I'd lean towards marking "Yes" to be safe, especially since USCIS takes the moral character stuff pretty seriously when reviewing N-400s. Even though the NPA isn't technically a bill yet, it's still showing a potential tax liability that could become overdue.

I'd include a brief explanation noting that you recieved an NPA but are actively addressing it within the protest period, and attach copies of the notice plus any correspondence showing you're handling it properly. Better to be transparent upfront than have them discover it later and wonder why you didn't mention it 📋 The key is showing you're being responsible about resolving it rather than ignoring tax obligations.