What is the exact model of the computer? You can get this information either by clicking the Apple menu and selecting "About this Mac", or by entering your serial number here or here.
What happens when you boot into Recovery Mode? Does it actually boot into Recovery Mode or does it boot normally into macOS?
Are you having other problems with this laptop which is why you are recycling it?
Is Filevault enabled on this laptop? If not, the I suggest enabling Filevault and letting the encryption process finish which can take days depending on the size & health of the drive. After that it makes erasing the drive much easier.
FYI, here is an Apple article on how to prepare to sell or dispose of your Apple computer (they assume Filevault is being used):
https://support.apple.com/en-us/HT201065
Edit: Also what version of macOS is currently installed?