I obtained the following sentences at this website https://www.esl-lounge.com/first-certificate/first-certificate-key-word-transformations-27-ans.php.
The picnic was cancelled due to the rain.
The picnic WAS CANCELLED BECAUSE OF THE rain.
The first sentence is wrong but the second is correct.
Why can't we use 'due to' in the first sentence?
Another source (from Magoosh) https://gre.magoosh.com/flashcards/grammar/right-or-wrong/the-picnic-was-canceled-due-to-rain suggests that the first sentence is wrong.
The reason provided by Magoosh is as follows:
"The picnic was canceled, because of rain" or "Cancellation of the picnic was due to rain" or "The cancellation, due to rain, was a problem for ...." The word "due" is an adjective, a noun modifier. It is allowed to modify a noun, as it does in the second and third structure here. It cannot modify the action of a verb, as the original has. For that, we need "because of," which opens a preposition phrase that can modify verbs.