Acer Aspire screen started flickering, external monitor is fine

one more on the software side: if the flicker happens mostly when you scroll or switch between light and dark pages, it can be the brightness adjusting to content. Settings > System > Display > Brightness, turn off "change brightness based on content". the intel or amd control panel has its own version of that too, sometimes called adaptive brightness or display power saving.

Restart and watch the screen while the Acer logo or BIOS is up. If it flickers there too, it's hardware, because no Windows driver is running yet. Usually the thin display cable that runs through the hinge, especially if it gets better or worse when you move the lid. If it's only in Windows, roll back or reinstall the graphics driver and check the refresh rate under Display > Advanced display. On Intel graphics, turning off Panel Self Refresh in the Intel Graphics Command Center has fixed flicker for a lot of people.