I am trying to enter a second email of mine as 'reachable at' but I get a message that 'this email is not available'. Can anyone help about this issue? Thanks in advance

If your e-mail address was associated with an ID in any way, for example as a Reachable at or 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.