Bitkarrot may have a more direct answer for ya.

I havent setup LNBits yet, but cursory glance at github repo implies an environment variable can be set to define LNBITS_ADMIN_USERS as a comma separated list of userids.

MyNode github repo defines env without any values in LNBITS_ADMIN_USERS.

It may be possible to modify the env file directly and restart the lnbits service. On mynode, the file is at /mnt/hdd/mynode/lnbits/.env