I tried to config iMX6ULL device tree and when Build U-Boot I get this

scripts/kconfig/conf  --syncconfig Kconfig
  CHK     include/config.h
  CFG     u-boot.cfg
  GEN     include/autoconf.mk.dep
  GEN     include/autoconf.mk
  CHK     include/config/uboot.release
  CHK     include/generated/timestamp_autogenerated.h
  UPD     include/generated/timestamp_autogenerated.h
*** Your GCC is older than 6.0 and is not supported
arch/arm/config.mk:66: recipe for target 'checkgcc6' failed
make: *** [checkgcc6] Error 1
make: *** Waiting for unfinished jobs....
make: *** wait: No child processes.  Stop.

of course i tried to upgrade gcc to a higher version but this happened when i wrote the command add-apt-repository ppa:ubuntu-toolchain-r/test

add-apt-repository ppa:ubuntu-toolchain-r/test 
bash: /usr/bin/add-apt-repository: /usr/bin/python3: bad interpreter: No such file or directory

any idea?

Thank you!

Please follow steps described here - Build U-Boot and Linux Kernel from Source Code | Toradex Developer Center

In case of further question please provide version of U_Boot you are building.