Reseting my Macbook to factory settings

I have a Macbook Air 2015 11 inch, I bought a new one with M2 chip, so I would like to reset the old one to the factory settings so I can sell it. I tried erasing all data in my Disk Utility app but the "erase" option is just unavailable, it's unclickable. Why am I not able to erase all data from this macbook? I saved all I need on the icloud drive and log on to my new device with my Apple ID credentials. I followed all steps for erasing my data on your website but the option is just not available.

Thank you in advance.