Appeal or certificate of error: two different remedies
One is for disagreement about value. The other is for facts that are simply wrong.
They are not interchangeable, and using the wrong one loses time you may not have.
An appeal
For disagreement: your value is too high, or you are assessed unequally. Filed with the Board of Review within the appeal window. Decided on evidence, and the board may sustain or raise as well as reduce.
A certificate of error
For a mistake of fact: a building that does not exist, a duplicate assessment, an exemption wrongly removed, a plainly wrong square footage. Available in some counties outside the appeal window, because it corrects an error rather than re-deciding a judgement.
How to tell which you have
Ask whether a reasonable person looking at the record and the property would say the record is wrong, or merely that the value is debatable. Wrong is a certificate of error. Debatable is an appeal.
Practical advice
If you have missed the appeal deadline and your complaint is factual, ask the assessor about a certificate of error before writing off the year. Availability and scope vary by county, so ask specifically rather than assuming.