Summary
When editing an alert subscription in Strategy Web 10.3, the following error occcurs

Could not display "alertEditor" using "AlertsSubscriptionEditStyle" as style

3. Set a condition.

4. Press OK.
5. Go to My Subscriptions in Strategy Web.
6. Press the Edit button.
Cause
This is a known issue with Strategy Web 10.
Action
This issue has been addressed with Strategy 10.4. Upgrade to Strategy Web 10.4 in order to take advantage of the fix.