Based on this thread, I would guess that you removed the EFI partition when installing your Linux system. You can safely ignore the message and suppress it by disabling 'HP Quicklook' from the BIOS setup.
This is the relevant information from the linked thread:
This is a feature. You are using info-button to start and have no
efi-partition (HP_Tools) installed.
Disable 'HP Quicklook' from bios setup.
HP EliteBook Notebook PC Series - BIOS Application Error (501)
You may have get a 'BIOS Application Error 501' error message if using
not HP Preload image and press INFO button when starting unit. The HP
BIOS application selected is corrupt or missing. Please install the
application and try again.
BIOS Application Error (501)
Enter - Continue
Details This error is the result of using the INFO button launch HP
EFI Application and if not using HP Preload image, the operating
system cannot find the EFI partition.
What is EFI ? EFI is a set modular interfaces that replaced the set of
traditional BIOS interfaces between the OS and platform firmware. The
EFI partition is viewable on HP Preload image and HDD and is labeled
as HP_TOOLS. The EFI partition and HP EFI application are not required
for the basic operation of HP Notebook, however, if partition is
missing, EFI feature is not available.
Read mode on EFI Features on the "HP Business Notebook Computer - EFI
Guidelines" that are downloadable on the www.hp.com
EFI Guidelines (116.5KB, PDF) HP Business Notebook Computer