If you keep the board and the schematic open the program will keep them consistent.
If you close one and make changes to the other you are in trouble.
The msg tells you what is inconsistent. If you have only made a change or two you may be able to undo them to get to where they are consistent. If not remove the board file and create a new one for the schematic.
After you do this once or twice you will lean not to do what hurts.