On GitHub, I found quite an old issue similar to yours. Make error · Issue #170 · gnab/rtl8812au · GitHub
Thank you for trying.
Do you have any recomendations for me to try?
Try make ARCH="arm64" instead of ARCH=arm
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Thank you for the tip and sorry for probably stupid question: where can I make ARCH="arm64" instead of ARCH=arm?
This was what they recommended on github 
I have to admin I never compiled something.
Normally this is an option for your make command.
Like sudo make ARCH=arm64 MOREOPTIONS etc etc ...
But also now I see this was already tried with no success: https://github.com/MichaIng/DietPi/issues/4387#issuecomment-1201557452
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I guess you start the build process somehow. Have a look if you can specify the option.
That is slowly a problem above my paygrade… 
Maybe I go with ARMv7 instead?
What command did you execute when this error messages came up?
I executed “make dkms_install”
Ok then try
make ARCH=arm64 dkms_install
root@Jedilnica:~# cd rtl*
root@Jedilnica:~/rtl8812au# sudo make ARCH=arm64 dkms_install
cp -r * /usr/src/8812au-5.6.4.2_35491.20191025
dkms add -m 8812au -v 5.6.4.2_35491.20191025
Error! DKMS tree already contains: 8812au-5.6.4.2_35491.20191025
You cannot add the same module/version combo more than once.
make: *** [Makefile:1789: dkms_install] Error 3
This is the result now.
So there is already a module (The not working one?), you have to remove this before compiling a new one and add it, I guess.
I found a similiar problem here.
They solved it with sudo dkms remove 8812au/5.6.4.2_35491.20191025 --all, but I would suggest to make a backup before, bc I have no experience with this stuff either and can not estimate how legit this is
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