Compression Campaigns are currently designed to prevent instant scheduling if there is a number selected for the target days other than zero.
If target days are selected as a positive integer and not "0", It works as expected and allows you to schedule for next day or next working day.
If target days are selected as a positive integer and not "0", you will not be able to select "Now" in the "Schedule For" field. The below error will be displayed.
NOTE: If target day is selected as "0", It will allow you to schedule the compression campaign immediately(now).