Your constraint is basically saying "the field accid in every record in the Customer table *must* match the field accid in *some* record in the Accounts table."
You can't add the Cutomer record, which needs an *EXITING* accid, until *AFTER* the Accounts record with that accid is added.
In other words, you have the order of your INSERTs backwards. Add to the Accounts table and *THEN* add to the Customer table.
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