I am a morderatly new user to the world of the Ubuntu MATE + Pi, but have a lot of experience with ubuntu MATE and the Raspberry Pi (seperately). I recently have been using steam link with the Raspberry Pi, and so I went about trying to install Steam Link. However, it said that steamlink was not found. I know for a fact it's supposed to install on ubuntu MATE, because I've read about it in their Raspberry Pi documentation.
I just checked the Software Boutique's curated index - which adds a PPA containing Steam Link:
Steam Link is only supported on armhf. There are no packages for arm64.
A quick look online suggests there are no arm64 builds for steam-link either, nor is there any source code to compile for arm64 yourself. I'm afraid the only option would be to re-install the armhf version of Ubuntu MATE and try again.