A Switchboard always needs at least one Admin. Make someone else an Admin first.
The change would have left the Switchboard with no Admin, so it was refused. This happens when the only Admin tries to become a Manager, or when another Admin changed roles at the same moment.
Go to Settings > Switchboard > Members.
Make another member an Admin first (or invite one).
Then change your own role.
Still stuck? Send AVX-2010 to hello@theavoxai.com along with what you were doing when it happened.