What's missing from the pics is the properties of the fields. While you might have every field matching in type, you might have Required set to yes in one or more destination fields, and you're trying to append Null values, which of course, you cannot and will raise that error. I would look for that or a similar situation first, because IIRC, appending one type of number into another (e.g. decimal into long) will work but the target field will accept only what fits. Thus, 123.456 appended into a long data type field will result in 123 as the decimal value won't fit, and the append won't raise that error.
EDIT: pretty sure it the number specified is for records and not fields. At least that's what my experiment indicates:
Code:
INSERT INTO test ( qty, amount ) SELECT "test" AS qty, "apple" AS amount
FROM test;
One field append or two (as shown) the message gave the same count.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.