Sounds like you're trying to use the clone stamp tool, but you haven't selected anything as your source. Try using a layer with content as the source and alt-clicking to define the source to copy/clone from first.

"Impossible to use buffer because the duplicate zone is not defined (alt-clic to define one source point)

This will likely be this error:

Could not use the clone stamp because the area to clone has not been defined (Alt-click to define a source point).