I assume you mean the packages like Character, Environment, etc. You may need to install them. I believe when you first install Unity you have the option to have these installed by default. You can reinstall Unity and only install those packages and uncheck everything else you already have installed.
I’m sorry, it’s 2 years later, but I had a same “problem”. Reinstalling unity didn’t help, there are no such options as standard assets (2D, character..), but I found them at Asset Store tho.