On laptops, the built-in display is generally handled by the low-power GPU. To capture that screen, you would need to set OBS to run on the low-power GPU. Unfortunately, external monitors are sometimes handled by the high-performance GPU. This would appear to be the situation in your case.
You will be unable to capture both displays simultaneously.
To select the GPU OBS runs on, here is the updated method (from Win10 1903 onward):
https://obsproject.com/wiki/Laptop-GPU-Selection-Windows-10
You MIGHT be able to do this using both the obs-ndi plugin, and NewTek's NDI capture client set to run on the low-power GPU, sending to OBS on the local machine, but that's the only workaround I can think of.
Additionally, Display Captures should be avoided at all costs in any case, unless there is a specific need to use one. DC is the least-performant capture method, and can cause major issues when included in the same scene as a Game or Window Capture (as you are), even if they are not active at the same time.