Hello Nonnutnon, thanks for joining the Intel Community.
The reason why you are not being able to set this custom resolution could be a limitation of the VBIOS.
Bear in mind that sometimes the VBIOS is limited to the OEM and in your case since the computer has switchable graphics; the setting of a specific resolution could cause conflict between the video card and the onboard video.
It is important to mention most of the time the system will be using the onboard video, but for example when you are gaming, they automatically switch to use the nVidia so the performance is highly better. Once you finish doing that task, the system will turn off the nVidia one and switch back to the integrated graphics.
I would like to know why are trying to set a resolution that you already are using? In your post, you mentioned that the current resolution is 1366x768 60Hz. Does it change at any time or are you having other resolution problems?