A sequence error means the register thinks it is in the middle of a transaction - or at least a part of the register thinks so. This can sometimes be caused by corrupt data or an abrupt power loss.
The Initial Clear procedure should eventually clear this error, although you may have to try it two or three times.
When programming, you simply need to set the PLU rate, and you can then program the tax separately.
If you cleared the PLU file then there is nothing to edit, so you will get an error.
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