STANC2 does not work in 2.26 as there were a few breaking changes that prevent it to work. Its now been removed on develop. It should still work with 2.25 if I recall correctly.
The other option is to use emulation for stanc3 compilation phase.
@StaffanBetner has used this. There is an issue for this: QEMU emulation of stanc3 on architectures without provided binaries · Issue #956 · stan-dev/cmdstan · GitHub