Should I do 20 minutes stair master everyday just to condition my cardiovascular system?

Usually lift weights 6 times a week and 5 times if im busy with school.

I lost like 2 pounds (probably mostly water) already but my lifts stayed the same. This has been going on for about a week now.

Why not...if you're just starting though don't bite off more than you can chew and burn out and relapse

Do whatever is going to ensure long term success

Switch it up don't do same machine tbh

20 minutes of HIIT fasted of the stepper ( i prefer stepper over stair master )
makes a massive difference.

1 minute moderate, 1 minute high intensity
for 20 minutes.

You should be doing more. Everybody should be.

why

never undermine the importance of cardio!

Yes

I recommend cardio exercises every day. If not everyday then at least 3-4 times per week minimum. 30 minutes per session would be better than 20.