Is it okay to pay only 15 years of social security?

If you are an employee's social security, the old-age insurance has been paid for 15 years, and you have just retired, you can only pay medical insurance.

If you have paid 15 years of old-age insurance and have not retired, you cannot pay medical insurance alone, because old-age insurance and medical insurance are tied together.

If you are a resident, you don't have to consider this issue and you can pay separately.

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