Why can't I enter bios?

Because each company chooses to use a different key to enter the BIOS/UEFI. 

The term BIOS is now a slang, despite that manufacturers have been using UEFI firmware for the last 8-9 years.

I believe the F2 key is the correct key. You'll want to tap, and keep tapping the F2 key right after your hit the Power On button.

It's not out of the ordinary for a system like yours to not want to enter the BIOS unless you're using a PS/2 keyboard. I would suggest trying it with your current keyboard a few times before thinking of trying with a PS/2 keyboard.

please see owner's manual for how to do this

If it's got a "UEFI" type bios rather than traditional, the whole thing is very different.

See the instructions here:

https://www.windowscentral.com/how-enter-uefi-bios...

Tap the F2 key to enter System Setup when the Dell logo appears. If you have trouble entering Setup using this method, press F2 when the keyboard LEDs first flash. Try not to hold down the F2 key, as this can sometimes be interpreted as a stuck key by the system.