When I was loading the modpack, It was at Initializing mods phase 2 before it crashed.

https://pastebin.com/QRX2WrTP

Caused by: java.lang.IllegalArgumentException: Failed to register dimension for id 7, One is already registered

Caused by: java.lang.IllegalArgumentException: Failed to register dimension for id 7, One is already registered

What does that mean?

It means what it is written. There is already a dimension with this id.
Try to reach who make your modpack or if it is a custom one, in config/AppliedEnergistics2/AppliedEnergistics2.cfg you should have something like that
I:storageDimensionID=7 replace the 7 by another value that doesn't collide with other mods
This is not an AE2's issue it's a configuration issue.

Okay, I found it, but it won't let me change it. All I can do is highlight it.

It's under spatialio.

It's strange that you cannot change the value
Which editor do you use ?

The top of the screen says cool file viewer.

If you cannot modify with this one try to open it with notepad

Okay, I opened it in notepad. What do I do now?

Same as before replace the 7 I:storageDimensionID=7 by another value that doesn't collide