Uable to check active assignation - more than one is found.

To fix this error you should check all Active trips of this Truck and make them delivered. 

This Error happens when we have team Drivers and when creating Trips we spleet team drivers, as result in Truck assignation system creates two lines for two drivers and both are open.

That is why you can not move trips via stages.

To avoid such error you should change Team Drivers after all their Trips are in Delivery Stage. So you Open First Driver account and via New Assignation button you spleet drivers correctly.