Sims 4 - Unable to start Initialization error at startup

I had the same issue, but found out that after installing the button still said install, so I did the following:

1. Go into regedit with admin rights and check the following path: HKEY_LOCAL_MACHINE/SOFTWARE/Wow6432Node/Maxis/The Sims 4

In there I only had on string value with the name GDFBinary and the value: D:\Games\Origin\Games\The Sims 4\__Installer\GDFBinary_da_DK.dll

What I then did was add the following:

2. Add string value of "Install Dir" with the value of GDFBinary except \__Installer\GDFBinary_da_DK.dll

So the value will be: D:\Games\Origin\Games\The Sims 4

3. Add a new string value with the name "Locale" and put in the value of whatever the locale of your GDFBinary is. eg. Mine is da_DK, yours might be en_UK and so on. I put in the value da_DK

4. Now I am able to run the game!

So to recap I now have the following three string values:

GDFbinary with value: D:\Games\Origin\Games\The Sims 4\__Installer\GDFBinary_da_DK.dll

Install Dir with value: D:\Games\Origin\Games\The Sims 4

Locale with value: da_DK

Hope it works for you