Why can’t I put a certain email address that I’ve used most of all before any other emails. Why is it invalid?

Are you trying to change your Apple ID to that email address? If not that, what are you doing when you get the error? More information would help.

Apple hasn’t published any guidance for using an address again. Normally, if your e-mail address was associated with an ID in any way, for example as a Notification email address or a Rescue e-mail address, then it can't be reused with another Apple ID for approximately one year. I have also seen posts that stated Apple told them there was a 30 day waiting period before the address could be used again.  You will need to get another e-mail address to use as your Apple ID or you can try waiting 30 days to see if that works.