The message is relating to the runuser Linux command not existing on your system, not to a permissions problem with Asterisk. NB this is not Runuser.
Why do you need to run on Debian 11, rather than installing the FreePBX distro on a bare machine, which is the recommended method for non-experts.
See Freepbx16 on debian11 user...sh: 1: runuser: not found for more.