Can someone please help

I just hooked up the denon avr-s760h. Currently just trying to play music thru Bluetooth.

I’m only running a front left, front right, and a sub. Fr fl hooked up to fr and fl outputs on receiver. Sub hooked up to sub 1 output.

I can not get the sub to play. If I put it on surround mode only the sub plays. So I know it works. When I put it back on stereo mode there is only a “FR” and “fl” level adjustment. So sub adjustment so it’s like the receiver doesn’t think a sun is hooked up.

I’ve been reading the manual and gone thru every setting and can’t figure it out.

I’m frustrated if anyone has this receiver please help