You need to install a permission plugin first. Then, you need to add the permission multiverse.portal.access.PORTALNAME, but keep in mind this permission has to be given for every portal you want players to be able to use.
Also, according to the GitHub site for the plugin, you will also need to give players the permission multiverse.access.WORLDNAME .Personally, I've never had to use this second permission before, but it you can use it as a troubleshoot option if the first permission won't get the job done. (You'd need both of these permissions if you add the 2nd one)
I hope this solves your problem. If you need any more help just lemme know.
My discord is Sgt_Tony23#7055 if you want to reach out to me on there.
