You must dump your BIOS from your own PS2. The 2021 dev builds are picky. Use BIOS dumper v1.0 on your fat PS2. Recommended BIOS versions for 1.5.0 compatibility:

Pcsx2 150 Dev Build — 2021 |top|

You must dump your BIOS from your own PS2. The 2021 dev builds are picky. Use BIOS dumper v1.0 on your fat PS2. Recommended BIOS versions for 1.5.0 compatibility: