Premiere elements 2020: Low quality in part of the project

 There is a green line on the time line just above the poor quality section. The rest of the time line has got an orange line and it plays in good quality. 

By @Nuria5C51

Did you mean the reverse of what you typed?

Timeline playback is merely a preview of your video. It's not how your final video will look. Your finished output will look great.

The orange line indicates a need to render your timeline for smooth playback. (It may not be serious enough in your case to require it.) But rendering your timeline (by pressing your keyboards Return/Enter key) will turn the orange line green and give you a cleaner, smoother preview.

This timeline render sometimes produces a low quality preview. But you can set it to produce a higher quality preview by going to the program's preferences (under the Edit menu on a PC) and changing the timeline render quality. 

Although, as I said, this is just a preview in any event. It has nothing to do with how your final output will look.