FCPx won't update via AppStore from

I have updated my Mid2014 Macbook Pro to Big Sur.

FCPx 10.4.x will launch but locks up with spinning rainbow ball.

The AppStore is not showing any available updates, but in searching I have found FCPx 10.6.3 there and tried clicking on 'Update". It appears to download the file, but finally says it cannot install.

Are there any suggestions to resolve or similar experiences? I urgently need FCPx to be working.

thanks,

Geoff