Open IntelliJ IDEA, go to the "File" menu → "Plugins" → [Or directly press Ctrl + Alt + S] Click the "Browse Repositories" button and enter "Scala". Select the Scala plugin to install it.
If you have restrictions on proxies or any other problems, you can download the plug-in to your computer from the link below and select the option “Install plug-in from disk”
https://plugins.jetbrains.com/plugin/?id=1347
P.S. . .