Attempting to update the Amethyst 1 database and stopping the upgrade process while it is busy, can result in a restart of the amtserver to fail to continue the update.
[2021-05-25 10:06:26.021] [Version 126.96.36.199]Error upgrading database: unsuccessful metadata update
cannot create index AMTCHATDATA_PK
This indicates that the amtchatdata table could not be added properly. then the next time the server starts, it tries to add it again and fails.
I have not been able to test if you can stop all the DB access data and try again.
Stop All DB access software and backup and restore DB, then try again.