Before applying the redactions, you should still be able to remove any marks you don’t want. Try clicking the unwanted redaction mark and pressing Delete, or right-clicking it and checking for a Remove or Delete option.
If you mean the Undo icon itself has disappeared, its location may depend on your Acrobat version and layout. You can check Adobe’s guide to marking and applying redactions for the current workflow.
Just be careful not to click Apply until you’ve reviewed all the marks, because applied redactions permanently remove the selected content. If you share whether you’re using Acrobat on Windows or Mac and which version, I can help narrow down where the undo option went.
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