hi my mac serial isnt working ?

If your goal is to check your MacBook's current support coverage, and you went to the following site that stated that the serial number you entered, was invalid, then you need to follow the steps in this support article instead:

• Update your proof of purchase - Apple Support

Are you using this resource to determine your Mac's eligibility for authorized service?

Check Your Service and Support Coverage - Apple

Find the serial number of your Apple product - Apple Support

You're going to have to provide a bit more information: Writing an effective Apple Support Commun… - Apple Community