Hi,
Thank you for posting in Symantec community.
Could you share the existing SEP 11.x verison and verison details of 12.1 where you wish to upgrade.
It's Embedded databse or SQL database?
Avaialble free disk space on the SEPM installed drive?
Did second attempt to upgrade the SEPM? Getting same error? Tried to run upgrade.bat file? location of upgrade.bat file: <local drive>\Program Files\Symantec\Symantec Endpoint Protection Manager\bin\upgrade.bat