The under the sink shutoff valve has 2 outputs (1 - sink and 1 - dishwasher). Want to temporary cap the dishwasher output of the shutoff valve. How do I cap it? Looking to cap it for a week at most.

That angle stop outlet feeding dishwasher is most likely 3/8" Comp. I assume you have flex line feeding your DW. You have several choices:

1. Go to Home Depot and buy 3/8" Comp brass cap. Remove DW supply line from the angle stop and install this plug on to the valve outlet
2. Remove double angle stop and install single angle stop
3. In case of emergency: leave the DW supply line installed as it was there with DW, coil it under the sink and plug the loose end with 3/8 IP plug. If this size plug is not available and you need to plug it right away - then remove DW line, install short 3/8" Comp x 1/2" IP water supply and plug the loose end with 1/2" IP galvanized plug

That would be 3/8ths copper feeding the SW correct? You could cut the lined and solder on a cap or simply crimp the line and solder the crimp shut. Good luck, Tom
Sorry Milo! Posting right on top of each other

Mmm. I get the impression that there is a single shutoff valve and hat's the reason for the cap.

This could be an opportunity to put in two ball shutoff valves for the dishwasher and sink. Capping then is as easy as turning off he valve and removing the handle to prevent accidental activation.

Mine uses tubing, but at the shutoff valve end it's soldered, but there are two shutoffs. One for hot sink and one for hot dishwasher.