Cache plugin download

Hi!

I’m not able to download a cache plugin (w3 total cache nor wp super cache). It’s saying “plugin is not allowed… contact support.”
I do have some kind on cache clearing button on my wp (“Flush cache”), but I can’t figure out where it came from?
My site is running reeeeaaaallly slow and I’m kind of running out of ideas what to do?

I appreciate any kind of help at this point. Thank you!