I can only guess that you clicked a box that answered YES to indicate that you had your insurance through the aCa marketplace insurance program. If you had insurance from the marketplace, then you probably had a subsidy and you would need get the 1095-a and complete the 8962 to balance out the subsidy.
Do not ignore the letter. Look at the letter from the IRS to see if they give a contact method for you to respond. If you did not have marketplace insurance for any month, then take action to infORM the IRS that there was an error in your submission and you did not have any marketplace insurance at any time during the year. On the other hand, if you had any marketplace insurance (even in you had other insurance), you should contact the marketplace insurance office and take action to get the 1095-a.