Some people can't see mental images

I don't think so.

Anecdotally I know someone with aphantasia and they report having an internal monologue.

I’m aphantasic and can hear internal monologue just fine.

From everything I've read about aphantasia, I'm certain it described me -- I have absolutely no ability to consciously "visualize" things in my head (I was also surprised to learn that some/most people sometimes experience a smell sensation if they recall smells, or "hear" music when they think about music or have a song stuck in their head, etc.)

Really the only thing in my head is my internal monologue. If I'm thinking about something I've seen, it's my internal monologue "saying," with words, physical attributes I remember about it. If a song is stuck in my head, it's my internal monologue (in my own voice) signing the lyrics or my own voice humming the tune in my head. No sensation of it being the original artist or the actual instruments, it's 100% my own voice in my own head.

I have a friend who says she does not have any inner monologue at all, and thinks entirely visually. I can't imagine! We're on the pretty extreme opposite ends of the spectrum of how we think, apparently.

Yes, that. Same here.

I still remember (semantically) that my parents laughed at me, when they told me to count sheep to fall asleep and I told them all I can see is darkness and white dots when I close my eyes. They probably thought I was joking.