Select is a new helper type only available to integrations. It’s like an input_select but the end user can’t change the selection options. They come predefined.
You probably have an integration that is using this new helper but are running an old version of Home Assistant that does not support it.
Interesting, that makes sense I guess! I’m on 2021.6.3 so not the latest and there’s a good chance one of the integrations I’m using is newer than that… It seems like a benign error so I think I’ll ignore it for now, and try updating later (don’t want to break things just yet).