I just started using Lemmy so I might be wrong, but I'd assume that spellchecking depends on your platform, so your browser or app you're using. Are you on pc?
Browsers nowadays have built-in or installable spellchecking, Chrome should be included (IIRC it's installed from their web store). Browsers on Linux and I think also Android can also use the system-wide installed spellcheckers; I don't think Windows has any such capability.