I'm trying to erase all data from an old macbook pro but I can only erase the "free space". How do I erase any data from the laptop?

If the old laptop uses a hard drive, then first enable Filevault and let the encryption process finish. Then when you use Disk Utility to erase the drive it will destroy the encryption keys making it impossible for anyone to access the data on the drive. Otherwise you would need to write a single pass of zeroes to the hard drive.

If the laptop uses an Apple SSD, then a simple erase using Disk Utility will destroy all data on the drive since TRIM will cause all unallocated blocks to be zeroed automatically. I'm not sure how a non-Apple SSD will behave even with TRIM enabled since I'm not sure that setting is recognized in Recovery Mode or from a bootable macOS USB installer.

Here is what you should do when getting rid of an old Mac (many steps must be performed before erasing the Mac):

https://support.apple.com/en-us/HT201065

"You can't cut off the branch you are standing on."

When using the Disk Utility inside MacOS, you can't erase the partition MacOS loads from.

If you boot from Internet Recovery, it deliberately creates 25 or more RAM disks for its temporary data, and leaves the disk completely free. From Internet Recovery, you can select the physical drive by its immutable hardware-name and completely ERASE the drive.