Brown leaves on new liliac bush
Thank you for attaching the image of your troubled lilac.
The browning I see in your image suggests the plant is short of water. I suspect the cause is the combination of a recent planting date and the unusual heat we’re having this spring. The root system has not yet extended into the surrounding soil, thus the lilac is running out of water rather rapidly.
I suggest you rig temporary shade for as long as it takes the shrub to recover. (That may require all summer.)
Also, make certain the entire rootball is evenly moist. Depending upon the size container it was in, you may need to water every several days through the summer.
And stop fertilizing.
See “Watering newly planted trees and shrubs” https://www.extension.umn.edu/garden/yard-garden/trees-shrubs/watering-new-trees-shrubs/.