If you want a certificate for a private network or localhost service, you can use a self-signed certificate—you can make one with
openssl or for example the generator at
Then you can add it in your own browser and the browser will accept it from then on.
A certificate authority like Let’s Encrypt is trusted by browsers and so the certificates that we issue have to be given only to the people who operate particular public sites—because if we give you a certificate for
irisnet.org, we’re confirming to all Internet users that you are the operator of
irisnet.org, which is not correct from the other Internet users’ own point of view.