The gimp docs: 3.12. Clone
Quick check list:
In the Tool options: Suitable brush and brush size. Opacity 100. Brush dynamics off.
Bottom of the tool options is Alignment.
None (default) try that first, works like a stamp. . Aligned, moves the source relative to the destination. Fixed, source is fixed, use as a stamp.
Registered, clones from layer-to-layer or another image, otherwise nothing shows.
In use:
Are you holding the ctrl key down and does the little ‘disabled’ icon disappear? (now there is a thought, are you using a Mac and does the cmd key work ?)
Move to the source and click, let go the ctrl key. Move to the destination and paint.
40 second demo: https://i.imgur.com/ySbuxHn.mp4
The other way is not to clone. Make a selection, copy the selection, paste the selection. Move to destination and anchor