Why can't I install custom content for the sims 4 on my pc

If you're copy / pasting them, then don't do that. Stop. Here's how it'll show up in the game:

.package files:

1.  When your custom content shows up in downloads, right click the file and select 'Cut'.
2. Then go to the 'Mods' folder (Documents> Electronic Arts> The Sims 4> Mods).
3. You'll see a file saying 'Resource.cfg' is already in it, but don't delete that! You need it for custom content to show up in your game.
4. Then right click and select 'Paste', and your cc file is now in the game.

FOR .zip FILES:

1. Right click on the zip file and select import. (My zip file looks like a purple book, a blue book and a green book tied together with a brown belt)
2. After clicking import you'll get a mini screen saying 'Import to' and a list of places to import it. Import it too your desktop.
3. Go to your desktop and there'll be a couple of .package files there.
4. Right click the ones which you think are your cc and select 'cut' again.
5. Paste them into the 'Mods' folder.

Hope this helped!