ORA-64768
KEEP file size string cannot exceed available DISK_RESERVED_SPACE string.
Cause
There was no more space for a KEEP backup piece. The total DISK_RESERVED_SPACE minus the space currently consumed by KEEP backups was less than the size of the current KEEP backup piece. The piece that caused the error was deleted.
Action
Use DBMS_RA.UPDATE_DB to increase the DISK_RESERVED_SPACE value of the database or move KEEP (archival) backups to other storage using the DBMS_RA.MOVE_BACKUP function.