Hello I am new to LFS and I noticed on the Formula BMW FB02 if I idle for a minute or two I can't accelerate anymore, until I go to the pit stop. Also if I don't go to the pit stops the car will slow down and eventually stop after a while of racing is this normal?

Seems like your wheel or gamepad axis aren't calibrated well and it gives a slight amount of throttle all the time - while you leave car standing in gear,engine heats up the clutch. Check the CT bar on F9 or F10 menu,most likely it is turning red after a while of your idling.

I use keyboard controls, how would I fix this?

With keyboard? No idea. Well,don't idle with a gear in,return to neutral... And don't go full throttle right away when you accelerate from standstill until you get some 30mph - also burns the clutch.

Or out of fuel?
Remaining fuel is shown on the car's display (on wheel) or press F12.
In setupscreen go to left most tab and change "fuel load at start"

I go into neutral when idling and the fuel is fine I think.

Is 40.9(?) Good because I think that's what it showed last time

Oh yeah, and when I'm racing and it does that it goes to 6 really fast

If you are idle for a while the car will auto stall, you have to press I to start it again.

if the engine is still running (engine noise) but car stops moving despite being in gear, then the clutch is dead.
Lift throttle while changing gears to avoid that.