I got an outdated client error when I tried to load it up. It says to please use Spigot 1.13.2
Luckily that's easy to solve. In the Minecraft launcher, before you hit PLAY, go to the upper right corner where it says Launch options. There, create a new gaming profile with 1.13.2 set as your version. The name doesn't matter, but I suggest naming it Spigot 1.13.2.
After this, go back to the News tab on the upper left corner, and this will bring you back to the opening page. On the right side of the PLAY button, press the small arrow and select the profile you created. If you log in with that, you can play on the server.