SDErrorDomain error 104 when trying to use external SSD as Ventura startup disk

I have a 2017 iMac with Intel processor that is running incredibly slowly. I read that I can run macOS on an external SSD to, hopefully, speed things up. I bought a Crucial 1T SSD for Mac. I formatted it to APFS with the GUID partition map, downloaded Ventura from the App Store and installed it on the external drive. In system preferences I have selected the external drive as the startup disk but when I try to restart I get error 104. Any suggestions?

Thanks.