This might be because you are trying to inserting a detail record that expects a header record to exist in another table.
For example, say you are trying to insert 'Union_1' into column in a table (employee?), but that table has a constraint where 'Union_1' MUST exist in table union in column unionname. If 'Union_1' does not exist in union, it is not going to allow you to insert it into the other table.
BTW, Union is a keyword so I'd be careful with using it as a table name.