Build monitor from laptop (no bootable device)?

I'm experimenting a bit and want to build a second monitor from my old laptop (fully functional only extremely slowly), I have all the parts in front of me.

If I now connect the screen to my new laptop, via HDMI, it shows me "no bootable device" (see pictures).

What do I do now so that I can use it as a second monitor?

(In tutorials, people ordered an extra controller for the monitors, but since everything still works fine for me, I hope I don't need something like this)

I just need the screen without a keyboard.

Lg

This will not work out, since the display of a monitor is usually plugged directly into the motherboard via a manufacturer-specific connector.

And with your tinkering here I would say based on the "error message", the box can't find a hard drive / USB stick with a bootable system?

The monitor is connected to the motherboard with a cable (flat connector).

The error no bootable device actually has nothing to do with the screen, but much more with the operating system or the BIOS.

The error says that the laptop's BIOS tried to boot into an operating system but was unable to find a bootable device.

You can't easily connect the monitor via HDMI. The HDMI port on your laptop will be an HDMI OUT port, which means that signals can be sent through the port to e.g. A projector or monitor can be directed. However, you can't use HDMI IN on the majority of laptops, which would mean that you use the port as an input.

In general, this would probably work even if only with the operating system or external controller, but not with your dry motherboard with display, as I imagine for you…