SYMPTOM:
Strategy Web users will notice the following error when trying to create a File subscription using "Send Now" as a schedule:
"Set a valid schedule before saving the subscription"

This only happens when using "Send Now" as a schedule as shown in the image above.
CAUSE:
The Strategy Web user does not have the necessary privileges to execute a subscription using "Send Now" as a schedule.
ACTION:
Grant the Strategy Web user the following privilege under the Deliveries section:
"Use send preview Now"

Log out and back in for this change to take effect.
Notice that this privilege was seperated in 9.3.1 as explained in TN43699 "Users are unable to use either "Send Now" or "Send Preview Now" functionalities in MicroStrategy 9.3.1"