You have to go way back to a Gimp 2.10.8 for both to work. I think seamless clone stopped working about Gimp 2.10.12
A quick show in Gimp 2.10.8 (appimage) https://i.imgur.com/HKq1UQT.mp4
Probably the closest to seamless clone at the moment is the gmic plugin and Layer -> Seamless Blend
N-point a bit of a disgrace
I have a tiny java app that does that and if you want a good example then Krita has a grid-deform
edit: ...also the gmic plugin Warp (interactive)
A quick show in Gimp 2.10.8 (appimage) https://i.imgur.com/HKq1UQT.mp4
Probably the closest to seamless clone at the moment is the gmic plugin and Layer -> Seamless Blend
N-point a bit of a disgrace
I have a tiny java app that does that and if you want a good example then Krita has a grid-deformedit: ...also the gmic plugin Warp (interactive)