Cannot create subfolders in Mods.

It may be an useless idea (it's a long shot), but there's no harm in trying. You say that Windows doesn't let you right click to make a subfolder. What if you try to make it through the Windows command prompt? It may give you an error or message on why you have this problem.

1.- Check the path of your mods folder. It usually is: C:\Users\YourUsername\Documents\Electronic Arts\The Sims 4\Mods.

Yours could be different. For example mine is E:\MyUsername\Mis documentos\Electronic Arts\Los Sims 4\Mods:

2.- Right click on the bottom left corner of your screen and choose "Command prompt (administrator)". Follow the following steps with care.

3.- You will start at C:\Windows\system32. You need to go to the path of your Mods folder.
If your mods folder is located in C: drive, write "cd\" (see image below).

If your mods folder is in another drive, write the letter and :, for example: "E:" (see image below).

4.- Now write the path to your mods folder, something like this:  "cd Users\YourUsername\Documents\Electronic Arts\The Sims 4\Mods". It will let you know if there's any problem, like it tells me in the first screenshot because I don't have those folders.

5.- Use "dir" to ask Windows to list the folders and files that are located in your mods folder.

6.- Use "md newfolder" to make a new subfolder in mods with the name newfolder. It probably will give you some error message.

Use dir again to check if the folder is there. If it is, it should be visible if you open your mods folder like you normally do.

Let us know your results, good luck!