How to add time-dependent actions in Lightning Process Builder?

Scheduled actions are only available in two cases.

On the object node, the process is set to "Start the process only when a record is created." All criteria will then have the Scheduled Actions option available.

OR

On the object node, the process is set to "Start the process when a record is created or edited", AND on the criteria node, the checkbox "Do you want to execute the actions only when specified changes are made to the record?" is checked. Only criteria on which this is checked will have the Scheduled Actions option available.

Check Yes under Do you want to execute the actions only when specified changes are made to the record?

More details you can find here.