In situations when a manager is on holiday, the common practice, is temporarily changing the specific employees' approver.
This can be done in Time > People's time off > Policy balances > Select the policy by selecting the relevant employees > Actions > Change approver. Like this:

You can then change the approver to the manager when they're back to work.
Either that or having one of the HR Admins temporarily approving the requests (perhaps with the second level manager confirmation) if the request can't wait for the manager to return from holiday.
Another thing that is commonly done, is enabling the permission to approve pending requests from other on the "Managers" group, so second level managers can approve the requests for their reporting lines:

In this case, the manager of the manager who is on leave, would be able to approve the requests that are pending approval!