IRS letter for “possible penalty”?

Just got a letter from the IRS today stating that I have an estimated tax penalty on a previous return, and that I have a "possible penalty" (no amount specified) unless I make quarterly payments.

All of this is news to me since I've always been an employee at large organizations and don't have other side businesses/gigs/etc.

I'm confused as to what the letter means? Since there's no amount specified, it's not like I can make any payments, or is there? FWIW I did change jobs and there was a part of the summer when I was unemployed (w/o income) between my old job ending and new job starting. I did take a look at my IRS account and it says I owe $0, and I did generate a tax compliance report from the IRS that says that I'm compliant and have been without any penalties for the past five years.

EDIT to add that there isn't any identifier for type of letter/notice that I can look up quickly online, hence why I'm asking here. Also, the letter itself looks legit as all the contact information and links check out.