Downgrade Catalina to High Sierra or Mojave

Hi guys, whats up?

So I just took my mac pro last edition last month and I've been having problem with that. My mac is restarting randomly. I read about that, Catalina is not compatible with a lot of music softwares.

I wanna make a downgrade to Sierra or Mojave but I cant do this because comes a message, the version of mac is not avaiable and I cant make it.

Could someone help me? Where I can find it?

Thank you.