Good Evening 
Tomorrow between 9am and 1pm (UK time) our Sandbox driver simulator will be out of action. This short period of downtime is in relation to upcoming improvements to the simulator which we will be testing.
We apologise for any inconvenience caused.
Update:
This testing has now been completed and the Sandbox environment should be working as normal.
We will conduct another session between 4pm-6pm (UK time) this evening where you may again experience that your orders cannot be assigned during this time.