Linu’s Favor is a campaign currency used in the Tyranny of Dragons(ToD) campaign. In the first 90% or so of progression you only need to get a couple which is pretty easy, but at the last two boons, you will need a combined 80 Linu’s favor to unlock them which will take some time.
There are two ways of getting Linu’s favour, the quickest way is to combine both of these. First, you can donate three different other currencies that you get while doing all things ToD related. You donate them to a cart in the middle of the Well of Dragons, next to Elminster, if you have the Rewards for the Hoard quest that will reward you with a Linu’s Favor when you reach 1500 points.