For Chromakey (Green Screen) to succeed an image must have high contrast between subject and background, and the subject can't contain the background color, which doesn't have to be green.
For example...
Changing the background of content that is not shot in that manner requires different software.
Place the new BG content on track 1, the green screened image on track 2. Click the track 2 FX icon, choose green screen, and adjust sliders for best presentation.
The result... 