When merging queries together, they have to be the same type. When creating a query, the query type determines which type of records are being grouped together. For example, if you are working with a Revenue query, the default output will be amount. When working with a constituent query, the default output will be name.
When merging queries together, both queries must be the same type so that the same record types can be grouped together. You can merge dynamic and static queries together, but you cannot merge constituent and revenue or event and group sales reservations.
Answer: To resolve, you would need to rebuild one of the queries to match the source view of the query. For example, if you have a constituent query and revenue query, you may want to rebuild the revenue query as a constituent query in order to merge the queries together.