A1278 covers many years and models of laptops (all non-Retina) so it is not very specific. The specific model designation can be found by entering your serial number here or here.
It appears you have some APFS file system corruption or a corrupt drive since it is having trouble decrypting the Filevault volume. While booted into Internet Recovery Mode (Command + Option + R) or from a bootable macOS USB installer, or while connected to another Mac with this computer in Target Disk Mode try running Disk Utility First Aid on the volume.
If the volume cannot be repaired, then you will need to erase the whole physical drive and restore from a backup or clone.
There is a good chance the hard drive in your laptop is failing (or possibly a bad hard drive SATA Cable). You can try running the Apple Diagnostics to see if any hardware issues are detected. If you have (or can create) a bootable macOS external drive, then you can check the health of the drive using DriveDx. You can also put the laptop into Target Disk Mode and check the health of the drive from another computer (Mac - use DriveDx, Windows or Linux use GSmartControl). Unfortunately the diagnostics don't detect many drive failures, but if you are interested I can provide instructions to use a Linux boot disk to check the health of the hard drive.
I hope you have a good backup.