Question: I have a manuscript under review at Nature Scientific Reports. The editorial system shows eleven reviewers invited but only one has accepted. Is this a problem? Is it normal?

Answer: 
I would add to the other answers here that during COVID, journals – and tenure review committees as well as other bodies that rely on 'voluntary' service - have struggled with peer reviewers beyond the norm. Many academics are feeling a bit overwhelmed with remote teaching, the virus, and other disruptions, and speaking both as a journal editor and somebody who holds mid-level administrative positions, this is pretty widespread at least in North America right now.
Yes, it indicates a problem:

* It could be that your paper is uninteresting, such that most people don't really want to spend the time to review it.
* Or it could mean that the editor is "missing" and identifying people who aren't in the same field as your manuscript.

There's nothing to do except wait, though.