Hi @mark_walton1 welcome to Zwift forums.
You may want to confirm with Elite, but you should be able to put an 8 speed cassette on that trainer. It’s likely that the 11 speed freehub body will be a smidge wider than an 8 speed cassette, so you need to install a spacer or two before you slide the cassette on there.
If you need a spacer, it’s important to use one with the appropriate thickness, so that the cogs line up where your derailleur expects them to sit. If it’s a Shimano-made cassette, chances are you’ll want to stack one 1.8 mm spacer and also one 1.0 mm spacer. Can’t say what you’d need for other brands of cassettes.
This is where contacting Elite would come in. Also - consider purchasing from a local Elite dealer and have them ensure correct setup.
We also have a guide for installing cassettes on this thread. The key point is to buy a cassette of the same gear range & brand as what’s on your bike.