Can't you pay social security yourself without unemployment certificate?

Never found a job. Where did you get the unemployment certificate? In this case, individuals are allowed to pay.

If the employer pays social security unemployment, it must register unemployment. Only after registering unemployment will there be an unemployment certificate, and then individual payment procedures can be handled.

Without unemployment registration, there will definitely be no unemployment certificate, and it will be impossible to make individual contributions.

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