This seems like an error vscode displays. You probably have the document language set to 'plain' css, which is why directives from tailwindcss like @apply don't get recognised.
Depending on your project structure and how you use tailwind e.g via postcss the directives should still be applied. But for good measure you'd want to check the install guide again.

To fix the vscode error you have to do 2 things.
First it's likely that you haven't installed the tailwindcss plugin for vscode which you can download (and read about) here.
Once you installed this plugin you need to change the language of the file. Here are also the docs to do this.
But it boils down to navigating to view->Command Pallet... and then type in Change Language mode, in this modal you'd then search for tailwindcss and hit apply.