Typically you get that message when you're trying to append a text value to a number field or a decmial value to an integer field, etc. Usually a data type mismatch.
It's a generic message meant to cover 4 possible reasons why an action query cannot act against every record that it should. Each potential is reported and a count given for the reason, but those that don't apply are reported as zero. Unless you know it's OK, you should probably cancel until you fix the problem, which could be at the table level or query itself.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.
Thanks! You were correct. I happened to accidentally change a field from text to number in the original database, so got that message when I tried to append. All fixed now.