Can't leave a group in O365 (I'm not an admin)

Dit you try to promote someone else with powershell or delete the group with powershell? As you are not a Admin for O365 please let an admin perform this.

You can use the https://go.microsoft.com/fwlink/p/?LinkId=616191 to add users and give theme a role

You can use https://go.microsoft.com/fwlink/p/?LinkId=616195 to remove roles from users and remove users from a group

if this does not work try to delete the group with https://go.microsoft.com/fwlink/p/?LinkId=616186. before you start with these commands make sure your powershell session is in Azure AD.

Also checkout the microsoft support page: https://support.office.com/en-us/article/Manage-Office-365-Groups-with-PowerShell-aeb669aa-1770-4537-9de2-a82ac11b0540