If you didn't yet, do this first ...
1. Check for Windows Updates and Install Them;
2. Update all Video Drivers;
3. Disable Real Time Protection or Exclude Folders on Anti Virus on the folders used to download the game and also the folder where the game is going to be installed;
4. Download The Sims 4 Realm of Magic All in One Customizable 18.104.22.1680;
The steps to install this version of The Sims 4 1.55 with the new Realm of Magic Game Pack [Anadius] are in the video above (even though the video was made for a previous version of the game, the procedure is the same).
Basically, you need to disable your anti virus (or exclude the download and installation folders from its real time protection), download the release, run the setup file, choose the language, the installation options (if you want a full or partial install etc) and confirm everything. Then, before playing, always make sure Origin is not running in the background and have fun!