You don't have to create a new TRUCK class, you already have the object you need in "item" so just select that
TRUCK truck = (from item in context.TRUCKS
where item.NUMBER == TruckNo
BTW I googled the error message and this was the first result
c# - The entity cannot be constructed in a LINQ to Entities query - Stack Overflow
It's much quicker for you to do basic research before asking a question as you can often find the answer yourself.
Also consider SingleOrDefault rather than Single in case there is an issue with your data.