Why was I marked wrong for 'S'il te plaît' and the alternative was 'S'il vous plaît'?

Hi Ian, 

This is because it is a conversation between a waiter and a customer, they don't know each other and wouldn't use the 'tu' form to address each other.

Take a look at the following Kwiziq lesson which gives a summary of the three kinds of 'you' in French -

https://progress.lawlessfrench.com/revision/grammar/tu-and-vous-are-used-for-three-types-of-you

 

I hope this helps!