Hi,
Downgrading a Windows 8 PC to Windows 7 is not recommended.
The created Windows 7 flash drive is not UEFI enabled and that is the reason why it is not being recognized. Some if not all HP PCs will not boot Windows 7 in UEFI mode. You will need to boot into the bios and disable Secure boot and fast boot then enable legacy mode.
HP Secure Boot Windows 8 Topics
If you need Windows 7 drivers then look here as there are not Windows 7 drivers posted for your specific PC model.
BTW -- UEFI enabled devices need to be inserted before power on to get recognized by the bios.