Rebel T6 exposure time in aperture priority mode with flash

Following on to what AnthonyL said...

When you select Av mode, you're telling the camera, "I'll set the aperture but I don't care what shutter speed is used." When Av mode is used with flash, it's how you get fill flash (sometimes called slow-sync). From page 112 of the manual (the section on Av mode):

In low light, the subject is exposed with the auto flash metering, and the background is exposed with a slow shutter speed set automatically. Both the subject and background look properly exposed with a touch of atmosphere (automatic slow-speed flash sync). If you are handholding the camera, keep it steady to prevent camera shake. Using a tripod is recommended.

Whenever you care about both aperture and shutter speed, that's what M mode is for. Set up your M mode with a shutter speed of 1/200, then use whatever aperture you want along with flash. Using slower shutter speeds gives more background.

Or if you don't care that much about either aperture and shutter speed, and just want a nice flash picture, there's P mode.

This is how it works on every camera model I know of (that offers Av or A mode). Fill flash is a powerful tool, so much so that many people pay considerably more to get a flash that supports high-speed sync (HSS) so that they can use fill-flash in full daylight without having to stop their lenses way down.

That said...

Long ago, Canon decided to accommodate those photographers who had no use for fill flash, and instead wanted to leave their cameras in Av mode when using flash. Check page 261 of your manual for Custom Function 3.