If the watermark will not remove through Design > Watermark > Remove Watermark, it is often because it is actually placed inside the header as a floating image or WordArt object. Double-click near the top of the page to open the header, then click around the page until the watermark itself becomes selectable. Press Delete. Repeat for different sections if the document has section breaks.
Also check whether the “watermark” is actually a page background, shape, or image anchored behind text. Go to Home > Select > Selection Pane and look for faded objects, WordArt, or pictures. Select and delete them there.
If the document is protected, in compatibility mode, or generated from PDF conversion, the watermark may be embedded into the page itself instead of being a normal watermark. In that case, try Save As > .docx first, then reopen and remove it again. If still stuck, the watermark may be part of the original page image/background and not separately removable.
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hth
Marcin