I can not write a new post

Hello.
As a beginner WordPress user, I recently set up multisite. Then I ran into many problems. (I couldn’t use some plugins on my main site)
I watched a video of switching from multisite to single site on youtube. I think I did, but I’m still facing a lot of problems.

When I click to write a new post, I get a blank screen. I’m running into problems with Glossary, Jetpack, and Yoast and many more plugins. (Blank screen)

How can I get out of this situation? Can you help me? As a beginner WordPress user, I don’t want to make any more mistakes. I would be very grateful if you find a detailed solution.

Thank you so much!