Enabling the driver to mark a task as failed in-app can be done in Configuration ⇒ Task Issue Workflow ⇒ Allow Driver to Mark Task Failed.
Click Save once the feature has been enabled.
Task Failure Reasons
Enabling this shows a list of selectable reasons in the driver app before failing the task.
Setting this up for both pickup and delivery can be done in Configuration ⇒Task Issue Workflow ⇒ Task Failure Reasons.
It's also possible to add a custom task failure reason.
Please note that in order to have Task Failure Reasons enabled, the Allow Driver to Mark Task Failed option needs to be enabled.