how do I remove a thumbnail from my document

If you are referring to an open multi-page document using Thumbnail view, then if it is a default word processing application, you cannot delete a thumbnail without clicking in that associated page, and then removing the contents on that given document page representing that thumbnail.

If you have a Page Layout form of document, then every thumbnail is a removable page, and you can right-click on that thumbnail and choose delete.

See Pages User Guide for Mac - Apple Support

Quick navigation in Pages on Mac - Apple Support