The company asked us to sign an agreement refusing to buy social security. Is it effective?

Strictly speaking, such an agreement is invalid and violates the labor law.

The government stipulates that enterprises must buy social security for each employee. The enterprise pays 12% of the total wages for the employees, and the employees themselves pay 8%. However, in order to save the expenses paid for employees, individual enterprises take advantage of the reluctance of some employees to pay social security and sign an agreement with them to voluntarily give up social security to shirk their responsibilities.

If you want to start paying social security, the company has no reason to refuse you! If you don't want to pay, this is exactly what the company wants!

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