Can I set a reminder to be sent to the employees if they have not validated their clock ins/outs?

Plans: Time, Advanced, HR Starter, HR Professional and HR Enterprise. 

Yes, if you activate the clock ins/outs validation,  you also have a setting that you can enable and allows you to set a date when a reminder will be sent via email as long as the employee has not previously validated them. 

You will find it in the same area where you activate the clock in/out validation. Follow this route and choose the date and time you are interested in: 

Configuration >> Automations >> Validations.