Try creating an ad from Ad Manager instead of boosting your post. The post that your trying to boost is most likely not compliant with ads (which could be due to a number of reasons)
Then you're probably doing something against their ToS. Like a copyrighted phone-case design. Try running an ad for a different product with different text.
The product is definitely copyrighted so Facebook isn't allowing you to run ads for it. Run a different product.
What's on the tee?I just try it to run ADS for a t-shirt and same thing happenad not approved.
I have a decent amount of facebook ads getting disapproved as well, mind to share us your ad image?
Maybe we can help to improve your approval rate.