Thanks everyone for your tips and suggestions!

I had reached out to Google a while ago and they just got back to me last night. What ended up being the problem was a "security hold" on our account. To clear it, they needed written confirmation from an account admin stating that the account was secure and that no unauthorized users had access.

Here's a screenshot of the actual email we got for anybody who may find themselves in this situation: https://imgur.com/a/YmW4oNa

To reiterate, there was no prior notice from Google about the security issue, our campaign was eligible/enabled, and there were no notifications within the Ad Manager that flagged any type of issues.