Hi Ahmed, setting it to required with a condition for number of days > 15, means that a value for the selected field is only required if the number of days field has a value larger than 15. 

From what I understand, you'ld like to set a minimum value, so actually a field validation. I would propose to do that with an automation rule with python code: 

raise UserError("The 'Number of Days' value must be larger than 15")

You can set the automation rule to only address records of the employee contract which are saved (trigger) with Number of Days
Hope that helps, if else let me know on hi (at) latus . ch
Thanks!