Normally you do not have to pay employees for unauthorised absences from work.
However, an employee will be entitled to be paid for periods of absence if their employment contract says so, or if you have previously agreed to pay them. The same can apply if it is normal practice in your business not to make deductions for absence, even if this hasn’t been formally agreed or written down.
You cannot generally require an employee to work extra hours unless their contract says so.
In practical terms, you may want to think about offering a degree of flexibility. For example, employees might value being allowed to take a few hours off (for example, to visit the dentist) and then make the hours up later. It may also be counterproductive to penalise employees for being absent when it is outside their control — for example, during a travel strike.