"The magazine from the federated server may be incomplete. Browse more on the original instance." - why does this appear and how can I turn it off? I cannot log in to other servers to post

It's because your instance pulls content AFTER someone joins the community in another instance. You don't log in to other instances to post, you can do so in your current instance.

@ABCDE it's just telling you that having subscribed to a community on another instance after it was created there may be older posts that are not present on your instance and providing you a link to the same community on the home instance if you want to look at older/not yet federated content.

Not sure if you can turn it off though.

That is a Kbin warning letting you know that you're connecting to another server other than the one you're using, and that the community/magazine on the server you're using might be a little out of date. It then offers you a link to check out the server directly, should you wish to.

You cannot log into other servers, since the account is tied to kbin.social for the time being, and if you want an account elsewhere, you will have to register one there.

I do not believe that it is possible to remove thst warning just yet.