0: Wrong forum. This is no design discussion
1st: Search the forum...
This was answered in many cases.
2nd: Check you are not using OneDrive or move the addons folder away from it to normal users directory.
3rd: Check the help pages here for "How to install addons" and check if your addons are in this directory.
4th: Search for the file "UserSettings.txt" or "AddOnSettings.txt" on your harddrives/Sdds and see where it is. In the subfolder AddOns the addons have to be. If It's on OneDrive 2nd applies!