This is a merge validation issue, and it can happen even when neither party has an incomplete Personal Budget.
This happens because of an issue where a service user started a Personal Budget workstep that the worker completed without any financial operations taking place, or where the worker added or made changes to finances and then used the Abandon purchase action next. The action's purpose is to make it easy for the worker to remove any finance done on that workstep without manually abandoning each proposed change individually. However, the code behind the action can still leave empty data linked to the proposed changes in the supporting Finance tables. This causes the Mosaic merge validation to assume there's still incomplete Personal Budget work to be done.
We can apply the Empty Care Package Changes fix script referenced in the Merge Person has failed due to a finance error article. However, in cases where this doesn't initially work because of some other linked data in related finance tables, we'll need to fix that data for removal before applying the Empty Care Package Changes fix.
If you are experiencing this issue, the first check you should make is to determine and verify that neither merge person candidate has any open Personal Budget steps. If either does, that should be dealt with in the usual way. However, if neither person has an incomplete Personal Budget step in progress, please raise a new case, providing us with the following:
The Source Person ID.
The Target Person ID.