Hi,

You can see intellisense errors but clicking compile/upload should work okay?

If we see intellisense errors its probably because you opened an example directly instead of clicking "open copy". Visual studio can't save any intellisense or projects settings when opening examples directly because of the permissions that are applied when you install the arduino ide.

You can run visual studio as administrator or use the "open copy example" option.

If the problem persists try to build which normally resets the intellisense other try switching to a different board and back to the uno again using the boards list

After it saves the intellisense config once per project it should be happy, normally its automatic.

Hope this helps