I can’t download macOS Big Sur it keep saying “you may not install to this volume because it is currently being encrypted".

It means that you have turned on FileVault and it is encrypting the contents of your drive, and you can not download a update while that is happening. Go to security and privacy in system preferences, and select FileVault and it will tell you how much time is left before it is encrypted. After that finishes, you can update your Mac to macOS Big Sur.