Hi. There’s an explanation here: Marketplace disabled in non-Community versions? - #2 by Colin_SonarSource and here: SonarQube v8.5 and Beyond: Where did all the plugins go?
Basically, commercial editions of SonarQube don’t support the marketplace anymore and the plugins developed by SonarSource are bundled in SQ, so you don’t need to download them. You can’t remove or update bundled plugins.
If you want to use a plugin from a third-party, you’ll need to download and install that plugin manually.