Putting the real correct IP into a server address doesn't make it save

I've wanted to make a server for me and my friends for a long time and I have the server, the IP, and everything else. I tried putting in the correct IP and got rid of the jumble of letters I had used to fill in the blank to create, but It wouldn't let me save. I'm on mobile, and every solution I can find on the internet is only for computers. If you can, please tell me an easy way to fix this or fix it. Thank you!