Why are my projects telling me to install different Unity versions?

I’m starting different projects and when I go back to open an existing one it tells me yo install a different version to open. Why is this and is it necessary? Or is there a workaround?

 

Don't keep using different versions for different projects then you won't have the problems!

 

I’m not purposely using different versions. When I close my project and reopen later it says I need to download a different Unity version to open.