This sometimes happens when the default Probationary period has not been set and a probation date has not been entered on the employee card. To specify a default probationary period navigate into Administration -> Company -> Settings -> Company Settings and scroll down to the Probationary Period section:

Once on this page complete the default length fields as well as the 'Auto populate probationary for new employee'. You also have the option to dynamically update the probationary period depending on the start date so the same period is always used.