I try and create a new league while importing a roster file, but apparently nothing happens? I try and hit continue, but nothing happens

Creating a custom roster has been impossible to anyone's knowledge for a while. I have a work around idea that I haven't fully tested but...

As long as you are fine with a fantasy draft to start your league...Just do that, start a fictional league in a mode where you can be commissioner...make sure to check Fantasy Draft in one of the settings tabs. Once created and before you advance to the draft, export your database. Edit the rosters (making sure to not change the Player ID). Everyone should be listed as a FA. I would also make sure to keep the same amount of players as your export. When done, import your database. All changes will be reflected in the Player Cards. When you sort ratings for the draft, they may not match up to the edits, but in the player card screen, they will. It's tedious, but its essentially the same thing as having a custom roster.