What the error means
You don't have permission means this login cannot perform the named action. Ask your school administrator to open Core Settings → Users & Privileges, open your user and grant the needed screen access. The message names the missing grant or screen when available. Repeatedly retrying does not add access. The institute owner's existing authority is unchanged.
Your session has expired — please sign in again means you need a fresh sign-in. If two people share one username, create a separate staff login for each person instead of repeatedly signing each other out.
A connection or server error explains what could not load or save and offers retry or support. A validation error tells you what to correct. Failed saves keep entered details where the existing form supports it; do not assume an unconfirmed save succeeded. Subscription, employee-status and verification holds keep their own instructions. Error messages do not display database queries, stack traces or passwords.