I am unable to remove an anchor in a calc sheet
Anchor for what? An image? You can either move the image to anchor it elsewhere or click on the image and delete it. If the image is hard to see, open Navigator and double-click on the named image, or each unnamed until that image is selected, then either move with arrow keys or press Delete key.
If you can’t select the image, then maybe the sheet is protected. Unprotect sheet by clicking in menu Tools > Protect sheet, enter password if required. then steps above.
(In addition to what @EarnestAl wrote in his comment:)
You can’t remove an anchor without removing the anchored object, but in a spreadsheet a visual object like an image or an ordinary shape can be anchored To Page using the context menu (rightclick) or the menu path >Format>Anchor. The anchor for an object anchored this way is not displayed.