My undergrad loans are in deferrment while I'm getting masters, they sent a letter "in place of the 1098-E", saying I didn't pay anything, what do I do?

If the letter confirms $0 student loan interest paid, there is nothing for you to do. 

You would only need to report undergraduate student loans on your tax return if you actually made payments in 2018. 

I would save the letter with your 2018 tax return to confirm that your loans were in deferment last year.