I'm going to guess that means you are putting it up on a basement wall, and I'm going to guess there's been moisture in said wall turning the wall too chalky for duct tape to hold.
Mylar creates a barrier stopping air from getting to that moisture, which will soon grow mold behind you mylar and then could spread to the plants pretty easily.
Never use mylar or anything similar on a basement wall directly.
If you can't space it out away from the wall and hang it or stretch it on a frame away from the wall, don't use it.
Any outside walls have this problem actually, even indoors in the house on an insulated wall. Walls breathe, people don't understand this.
Use flat white Behr ultra pure white wall paint if you can't fix it another way. It's as effective as mylar that isn't perfectly flat and more effective than even slightly dirty mylar.
Either way you should use a sealer on that wall before anything, be it paint or whatever. One made specifically to seal basement walls.
my 2 cents