From error message, the SQL is failing due to existence of the table it is trying to create - PC_ASSIGN_WORKLIST. Not sure why the script is trying to create this table which should already be present in the existing schema. Could you explain further on the steps you followed so far and your upgrade approach?
try to manually generate and apply ddl changes. Pega provides 'generateddl' script to do this. Refer upgrade guide for steps to follow. As mentioned in the error message above, cleanup schema before running the upgrade scripts again.