what should I do.
There is not much you can do at this point. Most screen "protectors" will damage important coatings on an Apple display. Removing them leaves a blotchy, non-reparable pattern on the screen that most people find beyond annoying.
The external screen on any Retina Macbook Pro made (Mid 2012 to present) cannot be separately replaced. That cover, the display itself, the outer shell, and the cabling are all one sealed unit. The only fix for a blotchy screen is an expensive new display assembly. And this is not something that your warranty covers.
Obviously, Spigen should have a safe removal process.
If that fails, or they can't recommend how to safely remove it, the cheapest display replacement costs (13-inch, parts and labor) I've see users post here are in the US$500-600 range. More for bigger screen sizes.
So your necessary conversation with Spigen and the seller of the cover will likely involve who pays for the repair or a new computer.