try install require package first
sudo apt install debhelper dpkg-dev dkms dh-modaliases

from rtl8814au.

Thanks for the quick help!!! 👍 But...it still doesn't work, error message as below(Kernel 5.0-rc6):

dpkg-buildpackage: info: source package rtl8814au
dpkg-buildpackage: info: source version 4.3.21.1-24835.20190115
dpkg-buildpackage: info: source distribution unstable
dpkg-buildpackage: info: source changed by jimmy jimmy@jimmy-alpha
dpkg-buildpackage: info: host architecture amd64
dpkg-source --before-build driver
fakeroot debian/rules clean
dh "clean" --with dkms,modaliases
dh_auto_clean
make -j1 clean
make[1]: Entering directory '/home/joana/Desktop/Wifi_Driver/Coolshou/rtl8814au/driver'
cd hal/phydm/ ; rm -fr /.mod.c /.mod /.o /..cmd /.ko
cd hal/phydm/ ; rm -fr *.mod.c .mod .o ..cmd .ko
cd hal/led ; rm -fr .mod.c .mod .o ..cmd .ko
cd hal ; rm -fr //.mod.c //.mod //.o //..cmd //.ko
cd hal ; rm -fr /.mod.c /.mod /.o /..cmd /.ko
cd hal ; rm -fr *.mod.c *.mod .o ..cmd *.ko
cd core/efuse ; rm -fr *.mod.c *.mod .o ..cmd *.ko
cd core ; rm -fr *.mod.c *.mod .o ..cmd *.ko
cd os_dep/linux ; rm -fr *.mod.c *.mod .o ..cmd *.ko
cd os_dep ; rm -fr *.mod.c *.mod .o ..cmd *.ko
cd platform ; rm -fr *.mod.c *.mod .o ..cmd *.ko
rm -fr Module.symvers ; rm -fr Module.markers ; rm -fr modules.order
rm -fr *.mod.c *.mod .o ..cmd *.ko *~
rm -fr .tmp_versions
make[1]: Leaving directory '/home/joana/Desktop/Wifi_Driver/Coolshou/rtl8814au/driver'
dh_clean
debian/rules build
dh "build" --with dkms,modaliases
dh_update_autotools_config
fakeroot debian/rules binary
dh "binary" --with dkms,modaliases
dh_testroot
dh_prep
debian/rules override_dh_install
make[1]: Entering directory '/home/joana/Desktop/Wifi_Driver/Coolshou/rtl8814au/driver'
dh_install core hal include Kconfig Makefile os_dep platform usr/src/rtl8814au-4.3.21.1/
find debian/rtl8814au-dkms/usr/src/rtl8814au-4.3.21.1 -type f -exec chmod 644 {} ;
make[1]: Leaving directory '/home/joana/Desktop/Wifi_Driver/Coolshou/rtl8814au/driver'
dh_modaliases
dh_installdocs
dh_installchangelogs
debian/rules override_dh_dkms
make[1]: Entering directory '/home/joana/Desktop/Wifi_Driver/Coolshou/rtl8814au/driver'
dh_dkms -V 4.3.21.1
make[1]: Leaving directory '/home/joana/Desktop/Wifi_Driver/Coolshou/rtl8814au/driver'
dh_perl
dh_link
dh_strip_nondeterminism
dh_compress
dh_fixperms
dh_missing
dh_installdeb
dh_gencontrol
dpkg-gencontrol: warning: package rtl8814au-dkms: unknown substitution variable ${modaliases}
dh_md5sums
dh_builddeb
dpkg-deb: building package 'rtl8814au-dkms' in '../rtl8814au-dkms_4.3.21.1-24835.20190115_all.deb'.
dpkg-genbuildinfo --build=binary
dpkg-genchanges --build=binary >../rtl8814au_4.3.21.1-24835.20190115_amd64.changes
dpkg-genchanges: info: binary-only upload (no source code included)
dpkg-source --after-build driver
dpkg-buildpackage: info: binary-only upload (no source included)
signfile rtl8814au_4.3.21.1-24835.20190115_amd64.buildinfo
gpg: keybox '/home/joana/.gnupg/pubring.kbx' created
gpg: skipped "jimmy jimmy@jimmy-alpha": No secret key
gpg: dpkg-sign.C5z5tcqa/rtl8814au_4.3.21.1-24835.20190115_amd64.buildinfo: clear-sign failed: No secret key

dpkg-buildpackage: error: failed to sign .buildinfo file

from rtl8814au.

The package did build successful. the error just mean signature error.
you can use following cmd to install the driver
dpkg -i ../rtl8814au-dkms_4.3.21.1-24835.20190115_all.deb
or you can build the package with following cmd to skip sign error
dpkg-buildpackage -b --no-sign

from rtl8814au.

Thank you very much for your help & support man!! 👍 However, I come to the last stage and find out - Kernel 5.0-rc6 is not supported yet... :(

14au-dkms_4.3.21.1-24835.20190212_all.deb
Selecting previously unselected package rtl8814au-dkms.
(Reading database ... 220602 files and directories currently installed.)
Preparing to unpack rtl8814au-dkms_4.3.21.1-24835.20190212_all.deb ...
Unpacking rtl8814au-dkms (4.3.21.1-24835.20190212) ...
Setting up rtl8814au-dkms (4.3.21.1-24835.20190212) ...
Loading new rtl8814au-4.3.21.1 DKMS files...
Building for 5.0.0-050000rc6-generic
Building initial module for 5.0.0-050000rc6-generic
ERROR (dkms apport): kernel package linux-headers-5.0.0-050000rc6-generic is not supported
Error! Bad return status for module build on kernel: 5.0.0-050000rc6-generic (x86_64)
Consult /var/lib/dkms/rtl8814au/4.3.21.1/build/make.log for more information.

CC [M] /var/lib/dkms/rtl8814au/4.3.21.1/build/os_dep/linux/rtw_android.o
/var/lib/dkms/rtl8814au/4.3.21.1/build/os_dep/linux/rtw_android.c: In function ‘rtw_android_priv_cmd’:
/var/lib/dkms/rtl8814au/4.3.21.1/build/os_dep/linux/rtw_android.c:615:62: error: macro "access_ok" passed 3 arguments, but takes just 2
if (!access_ok(VERIFY_READ, priv_cmd.buf, priv_cmd.total_len)){

Make Error Log:

                                                          ^

/var/lib/dkms/rtl8814au/4.3.21.1/build/os_dep/linux/rtw_android.c:615:7: error: ‘access_ok’ undeclared (first use in this function)
if (!access_ok(VERIFY_READ, priv_cmd.buf, priv_cmd.total_len)){
^~~~~~~~~
/var/lib/dkms/rtl8814au/4.3.21.1/build/os_dep/linux/rtw_android.c:615:7: note: each undeclared identifier is reported only once for each function it appears in
scripts/Makefile.build:276: recipe for target '/var/lib/dkms/rtl8814au/4.3.21.1/build/os_dep/linux/rtw_android.o' failed
make[2]: *** [/var/lib/dkms/rtl8814au/4.3.21.1/build/os_dep/linux/rtw_android.o] Error 1
Makefile:1553: recipe for target 'module/var/lib/dkms/rtl8814au/4.3.21.1/build' failed
make[1]: *** [module/var/lib/dkms/rtl8814au/4.3.21.1/build] Error 2
make[1]: Leaving directory '/usr/src/linux-headers-5.0.0-050000rc5-generic'
Makefile:1730: recipe for target 'modules' failed
make: *** [modules] Error 2

from rtl8814au.

I do not have kernel 5.0 but make some update according other's suggest
try pull the last code and build the driver first(check if there are any other errors)
git pull
make

from rtl8814au.

You are the Greatest man!! 👍 👍 👍 🥇 New ver. solved all the problems!!! Tested Ok on Kernel 5.0-rc6!!

Thank you again coolshou!! Appreciated for all your contributions and help!!

from rtl8814au.