The SELECT
clause includes a single field followed by a comma.
SELECT qryOutgoings.[Unique Number],
If that comma is also present in the actual query, discard it. However I suspect that stray comma is in the question but not the actual query because otherwise you would get a different error message.
Try explicitly casting qryOutgoings.[Unique Number]
as string.
INSERT INTO Exceptions ([Unique Number])
SELECT CStr(qryOutgoings.[Unique Number])
FROM qryOutgoings LEFT JOIN tblExlcusions ON qryOutgoings.[PLAN CODES] = tblExlcusions.[PLAN CODES]
WHERE (((tblExlcusions.[PLAN CODES]) Is Null));
If that doesn't eliminate the error, examine what Access sees as the data type of that field, and the length of the text it contains.
SELECT
qryOutgoings.[Unique Number],
TypeName(qryOutgoings.[Unique Number]) AS type_of_unique_number,
Len(qryOutgoings.[Unique Number]) AS length_of_unique_number
FROM
qryOutgoings
LEFT JOIN tblExlcusions
ON qryOutgoings.[PLAN CODES] = tblExlcusions.[PLAN CODES]
WHERE (((tblExlcusions.[PLAN CODES]) Is Null));
If nothing else solves this, try asking for the left-most 50 characters.
SELECT Left(qryOutgoings.[Unique Number], 50)