One of the possible reasons can be customizations in rule tables. Since new rule tables have been added in 7.1, the customizations in old rule tables should be applied to the new tables as well.
This issue can be resolved by carrying out upgrade in a multi-step process.
1. Use generateDDL.sh to create the upgrade DDLs
2. Apply the DDLs in the new schema
3. Identify customizations done on the rule tables like pylabel changes or pzpvstream changes
4. Reapply the customizations in newly created rule tables like PR4_RULE_RULESET, PR4_RULE_DECLAREPAGES etc.
5. Run upgrade.sh by setting bypass.schema.generation to true