It means that City field does not accept NULL values ;)
Please, folow these links:
10 tricks for handling null values in Microsoft Access[
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Common Errors with Null[
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Create a table (MS Access)[
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[EDIT #1]
Another solution is to define the order of fields:
INSERT INTO TableName (Field1, Field2, Field3,... FieldN)
VALUES(...)
Probably, you're trying to insert values which is not acceptable for this field.
Remember, date fields must be preceded and ended with
#
sign.
INSERT INTO TableName (StringField, DateField, IntField)
VALUES('myString', #2013/05/26#, 1)
[/EDIT]