As you may already know, Mojang intends to release 1.13 on July 18th. As andrewkm has stated in the ECC Discord #news channel, ECC will not be 1.13 compatible for a few weeks.
You have to use Minecraft 1.12.2 to connect to ECC. If you have already updated to 1.13, you will need to create a 1.12.2 profile to join the server.
If you are unsure how to create a 1.12.2 profile, you can use one of the following tutorials depending on your Minecraft launcher version:
1. Select the options menu in the top right corner.
2. Add a new profile.
3. Select the version "release 1.12.2" and save the profile.
4. Select the new 1.12.2 profile and start the game.
1. Select "New Profile".
2. Select "Use version: release 1.12.2" and save the profile.
3. Select the new 1.12.2 profile and start the game.
Once the server is ready for 1.13 you will be informed via the forum news section.