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What is missing from your screenshot are the layers.
Best guess, you are applying heal-selection to an area of transparency in a layer over the source.
Don't. Apply the plugin to the actual area.
edit:
And for users who do not have resynthesizer installed see: https://www.gimp-forum.net/Thread-Resynt...or-Windows which includes a video on installation and usage.
What is missing from your screenshot are the layers.
Best guess, you are applying heal-selection to an area of transparency in a layer over the source.
Don't. Apply the plugin to the actual area.
edit:
And for users who do not have resynthesizer installed see: https://www.gimp-forum.net/Thread-Resynt...or-Windows which includes a video on installation and usage.