This is happening in both Classic and Modern.
Clarity allows deletion of all cost plans in an investment, but when attempting to delete a cost plan, the following error in the APP log shows:
WARN 2021-09-21 10:34:31,584 [http-nio-8080-exec-53] niku.xql2 (clarity:username:session:revmgr.deleteCostPlans) Lock financialPlans held by user 0 requested by user 5000000
WARN 2021-09-21 10:34:31,589 [http-nio-8080-exec-53] niku.xql2 (clarity:username:session:revmgr.deleteCostPlans) Failed to delete records from query referenced in select deleteODF :The plan is locked and no updates can be made.
WARN 2021-09-21 10:34:31,605 [http-nio-8080-exec-53] caching.SecurityObjectCache (clarity:username:session:revmgr.deleteCostPlans)
Invalid security object key: INV_IDEARECORDPACWARN 2021-09-21 10:34:31,810 [http-nio-8080-exec-53] niku.union (clarity:user:session:revmgr.deleteCostPlans) was not found in file: messages_en.properties
Release: 15.9.3, 16.0.1, 16.0.2
Go to http://server/niku/nu#action:security.locks in the URL.
Does the Plan Lock show up? If so, remove it.
If this is missing, check if it exists in Administration > Lookups > Lock Type
If it is missing, recreate it manually.