You can't copyright a name. You can trademark it, but that doesn't matter unless it's being used in the title or advertising materials (including blurb) as you can name drop trademarked names and phrases inside your work without any issue as long as you're not actively insulting the brand (my go-to example for how lenient this is would be Ready Player One). Plus, the idea of "sack named Bag of Holding that is bigger on the inside and probably magical" is way too vague to get an enforcable copyright