The modifier part has nothing to do with the prerequisite part. For the modifier part, it has to do with the width of your display size. I’m guessing you were using a mobile device like a phone? Turn your phone sideways and the button to add a modifier will become visible due to the wider display size.
When adding a prerequisite, the options are faaaaaar too restrictive.
Proficiency only accepts armors, and "Custom Value" accepts the non custom respone of "the ability to cast a spell".
I also was not able to add a Modifier as a result. The + did not provide a button with which to add one like the other options did.
The modifier part has nothing to do with the prerequisite part. For the modifier part, it has to do with the width of your display size. I’m guessing you were using a mobile device like a phone? Turn your phone sideways and the button to add a modifier will become visible due to the wider display size.
Creating Epic Boons on DDB
DDB Buyers' Guide
Hardcovers, DDB & You
Content Troubleshooting