A few people in your exact situation have posted similar issues. If I remember correctly, had to do with something like the new tesla insurance is not sending proof of insurance digitally. Here are a couple of threads I found on the subject:
Have Tesla insurance, just got DMV letter "Intent to Suspend" for lack of insurance
Tesla Insurance Major Oversight
Have Tesla insurance, just got DMV letter "Intent to Suspend" for lack of insurance
Tesla Insurance Major Oversight