Sorry, encountered the same message admin is not in the sudoers file. This incident will be reported. And following your discussion I still wonder what is recommended way to remediate? In contrast to @sssammm I didn’t do anything to the system yet, and my state came from plain recover of backup to another VPS instance and setting SSH port since the new hosting prohibit 22 in spite of another port.
As in my case su doesn’t work as well, looks like I will need to redo the migration. But I still need to know what to do to empower admin user again. Should note, that previously there was no such problem after restoring on new instance.