You might have to subscribe to that channel first. If you go to that community where you get the sign in page, there is a box that says "to post in this community, type in xyz to the search bar" then you just paste what they say into whatever search bar is on the website you're logged into and you can subscribe

For example I am on lemmy.world but there is a community "ich_iel" I want on the German language version "feddit.de" so I have to go the community I want (ich_iel on feddit.de), find the link from the community, then type it in the search bar on Lemmy.world and I am able to subscribe and post in the community

It will be a format like !community@websitename

In my case I had to post something like "!ichiel@feddit.de" into the search bar on Lenny.world

I am new here so this is all I know