There are times when you don’t have control over the background. Try to keep it simple, but when it is busy, watch what is in the background. This is a sample (not my photo) where the photographer missed the obvious. However, I have made this same mistake with my background as well.

I found this photo on Mr. Bob’s Middle and High School Art Room Blog. It no longer looks active, but here is the rest of the content for bad portrait examples.
Mr. Bob's Middle & High School Art Room: Bad Portrait Photography (mrbobart.com)