This error proves that you cannot assign the chairman status to two people anymore, which was the purpose of the modifications.
Changing the chairman has become a two steps process:
1. Change the True (-1) value from the column Chairman to Null for the row containing the ancient chairman.
2. Set the same column to True (-1) for the row containing the new chairman.
Sorry for being dense! You can tell I'm new to all this.
OK, I understand what's happening but a one step process would be easier for the end user (who isn't me!). In other words, selecting a new Chairman by ticking their checkbox would automatically deselect the old Chairman. Is this possible?
PS It appears that I can't use a checkbox to make this selection because clearing it gives 0 instead of Null which fails the validation rule??
Another approach which I'd favor, would be to generalize your process by creating a table of Officers.
The table has 2 fields, name of office and person (or person ID).
In the table you identify in separate records the name (or person ID) of the Chairman, Secretary, Treasurer, Board Member 1, Board Member 2, etc. Then you can only have one name for each office, and you can do a lookup for any reports or on-screen forms.
Later as you get more sophisticated, you could add term-of-office dates for each, and eventually have a multi-year history for each office.