Why doesn't "usermod" work under superuser?

The usermod command successfully through sudo. sudo usermod -aG some_group some_user However, if you execute it through su, then there will be an error. su ...I enter root password... usermod -aG some_group some_user bash: usermod: command not found Why it happens? How to configure my system? The fact is that I have a script that I cannot change. This script uses command usermod under su.