You've dismantled the body of a car after an accident, distributed the pieces around the yard, and are now putting it back together.
Here it is.
But where is the right door?
Maybe it's under the left side panel. Or maybe Harvey carried it off to be painted.
I think the point of the lesson is to show that 'rechter-' is an adjective added to words to mean 'on the right side', as opposed to 'rechts', 'rechtsaf', and other words which are learned. It can be used with other words too, like 'rechterkant', 'rechterhand'. I guess it is included in the adjectives section because really it is an adjective, but one that is compounded with other words like a prefix. Therefore, the sentence does make sense as a didactic tool.
Maybe the right-hand door is a secret/hidden door and the speaker is unable to find it without asking where it is? Who knows, but I don't think the sentence is too strange situationally either!