To fix this delete the union and re-create it. The PCB panel can be used by setting it to Unions at the top) to check for duplicates and/or to find the one mentioned in the error.
If you try to move a union and get a message that this object is part of a locked union, you can check for locked objects using the PCB List panel
Select the items of the union you wish to investigate and access the PCB List panel. In the upper left corner of the panel, set the options to: Edit | Selected Objects -'include'- all types of objects. Check the "Locked" column to find the items in selection that have a 'Check' enabled for its locked property.