To spectate, you need to own at least the track and one car that is in the session.
When you choose a car from the drop down, select any car you own. From there, you will get the option to watch/spectate.
At that point, you can watch/spectate ANY car, even if you don't own it.
The only way to watch a car you don't own, is by selecting a car from the drop down list that you DO own.
You can watch/spectate by going to Racing on the left menu bar, then Spectate.
If you see a green check mark, this means you are eligible to spectate those sessions.
The red cart, means you will need to make a purchase to spectate those sessions.
Choose your event, then click VIEW SESSIONS.
On the sessions page, scroll down and click Watch for the session you want to watch.
You will then see a pop up to confirm, click WATCH.
Or, if you need to purchase content, it will prompt you to ADD TO CART the content you need and you can then complete the checkout process.
Once the needed content is purchased, go back and you will then be prompted to WATCH.
If you choose to GHOST drive, you will be driving the originally selected car that you DO own, not the car you are spectating (if it's a different car).