He won't let me in and I don't know why, but could you please help me whenever I want to come in, he won't let me.
Hi, what error message? You need to be running at least version 1.16.5 to join.
Yes I know, but I run it from version 1.17 I think there should be no error but it tells me this: Session not valid.
Have you tried logging out of the Minecraft launcher (top-left corner), then logging back in?
Does this happen when you connect to other servers? Make sure you're using an un-modified version of the Minecraft launcher from https://minecraft.net/download.
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