Would you please check on the source zip file to see if it should include a zen/argon2.cpp file? The Makefile suggests such a file should exist. This is for version 12.0
cppFiles+=../../zen/argon2.cpp
make[2]: *** No rule to make target '/tmp/FreeFileSync_Make/ffs/src/../../zen/argon2.cpp.o', needed by '../Build/Bin/FreeFileSync'. Stop. make[2]: Leaving directory '/usr/src/freefilesync/12.0-1/FreeFileSync/Source'
Ah, what you misunderstand is that when Zenju says it is fixed, he has updated the FreeFileSync_12.0_Source.zip file available for download. Just download the latest asset and it has it.