No, there is no free battery replacement service for the iPhone 12 – a phone that would now be many years out of warranty.
There are no active Apple Service Programs of any kind for that phone. I only see one active service program for any iPhone. It is for the iPhone 14 Plus, and doesn't have anything to do with the battery.
You say that you cannot afford a battery replacement, but if you continue using a phone with battery health that is degraded that far, the battery may swell and damage the phone, causing repairs to be more expensive - or making them so expensive (relative to the value of the phone) that you will need to buy a new phone. If I am not mistaken, your iPhone 12 would have displayed a notification that your battery needed replacing the first time that the battery health dropped below 80%. After which a "Service Needed" type display would have remained in Settings.
For reference: It is normal for battery health to drop 1% or 2% per month. Apple sold the iPhone 12 from October 2020 to September 2023. So your phone could be anywhere from 37 months to 72 months old. If your iPhone 12 still is on its original battery, one might expect battery health to be much lower than 72% by now. I do not believe it would be unusual for a 12-series phone to need two battery replacements (or to be approaching that point) by now.