I have macOS Sierra 10.12.6 and am trying to upgrade to Catalina but every time I try it says that it can't be done on this computer?

That depends.

Exactly what model Mac do you have.

There are 3 things that will usually get in the way of upgrading:

1. The machine is not compatible with the OS you're trying to install.
2. There is insufficient disk space available
3. The machine does not meet the minimum RAM requirements.

Does your computer meet these requirements for Catalina?

https://support.apple.com/kb/SP803?locale=en_US