What should I do after the social security payment reaches 15 years?

Paying 15 is only one of the necessary conditions for retirement, and paying 15 does not have to be stopped. If it is really impossible to pay because there are no units or individuals, you can go to the local social security agency to handle the deferred payment procedures, and then go through the renewal procedures of the old-age insurance when you have work or personal conditions.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.