Keyboard shortcuts Services not working

tl;dr: Try editing an existing shortcut first, then add a shortcut

I had the same problem trying to add a shortcut to a service (newly created with Automator) that didn't already have one. When I clicked the [add shortcut] button, instead of giving me a text box as expected, it would mystifyingly scroll the selected item out of view.

I tried it several times and was about to give up when I selected another item that did have a shortcut already and clicked on the shortcut keys. It gave me the expected entry box, and I clicked away to cancel. Then I clicked [add shortcut] again on my new item and it worked. I'm not sure that's what fixed it, but maybe if you try that it'll work for you.

System Preferences > Keyboard > Shortcuts > Services :

Select the Service you want, you should get an option to add shortcut :

Click on add shortcut, then enter the key combination you want to use for your chosen Service :

What keys, or combination of keys are you trying to use ?

Are you using a modifier key (Shift, Control, Option or Command) in your combination, when you try to enter your shortcut ?

What combinations have you tried ?

For example : Modifier - Number, Modifier - Letter, Modifier - Modifier - Number, or Modifier - Modifier - Letter ?

Don't know. I have only had problems with key combinations that have not worked, and needed redoing.