Make sure that ampinstmgr version prints out v2.6.x.x, and if that checks out, check your disk space df -h.
https://discourse.cubecoders.com/t/how-to-update-amp-to-the-latest-version/2297
If that checks out, restore the intsances.json in /home/amp/.ampdata using a copy of one of the latest instances.json-xxx.bak files, then reboot.