When rounding to the nearest thousand, you look at the following number to know if you go up or stay. In the case of 9,991:
9,991 If the 2nd largest digit is ≥ 5, then round up.
10,000
Therefore, you would round 9.991 to 10,000.
Hope this helps! :)
Edit: I literally solved for the wrong thing earlier lol; fixed it.