Discourse, our present forum platform, has outstanding strengths, but does not include a spellchecker. So how can you spellcheck your posts?
If you are asking, it is because you are using the Chrome browser on a computer to post Why:
both Android and iOS come with an integrated spellchecker
both Firefox and Safari browsers come with an integrated spellchecker.
I find the native spellcheckers to be solid, although not dazzling. Obviously, one possible solution is to use Firefox (or Safari) to post.
But there is a much better option! On Chrome, I use the browser extension Grammarly. it spellchecks me on the fly, and with more power than any native spellchecker I use. I am very pleased with it. I use the free version, btw – it is enough for me.
So there you go – if you post with Chrome, the free Grammarly extension will give you excellent spellchecking