I get these questions a lot.
Your mod load order is the issue. Open the Mods menu in game. When you mouse over a mod, arrows appear on the left allowing you to change its position. From top to bottom, your mods should go

1. 5X - The Ultimate Balance Mod
   
2. Unofficial Code Patch
   
3. Diverse Specials and Massive Planets
   
4. 5X UCP Compatibility Mod (5XCPC)
   
5. All other mods

This indicates you have not properly installed the Unofficial Code Patch.
UCP has to modify the game files in a way not possible through the standard modding system in order to do what it does. WhatIsSol has put together a java installer to make it as easy as possible, but you still have to find and run it. Assuming your using the Steam version of the game on Windows here's step by step instructions for those having trouble:

1. UCP uses a Java installer, so you need to install JRE[www.oracle.com] if you have not already done so at some point. If you're on Windows and don't know which to choose pick the Windows-x64.exe version.
   
2. Subscribe to 5X, UCP, and 5XCPC on their Steam pages.
   
3. Locate UCP on your computer. From your Steam install folder which by default is in Program Files (x86), navigate to steamapps\workshop\content\298050\1109736522
   
4. Extract UnofficialCodePatch-Release.zip to a separate folder (it's important you don't extract it to the same folder as the zip is in)
   
5. With MoO not running, run javaInstaller.bat and select the install option. You should see it copy a bunch of files, after which it'll say Operation Completed Successfully and you can close the program. UCP is now installed.
   
6. Run MoO. You should see the UCP version number in the top right of the main menu with the game version; if so this confirms UCP is installed correctly. Open the Mods menu. You should see 5X, UCP, 5XCPC, and UCP steam workshop. When you mouse over a mod, arrows appear on the left allowing you to change the order. Put the first 3 mods in the order
   5X
   UCP
   5XCPC
   It doesn't matter where UCP Steam workshop goes in the load order or whether it's active. Only the 3 mods listed above matter, and they have to be active and in that order. You have to re-run the installer any time UCP updates.

The UCP readme has instructions for non Windows/Steam as well.
Custom 5X Assets had new assets added in a recent update. You need to re-run the java installer for that mod.