In practice, it is very rare not be able to send this..
Could you please tell me if the order of the words in
not be able to is correct?
Also, should I add
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There are two infinitives, be and send, in the ungrammatical sentence
and both infinitives need a to to mark them. Send already has to, but be does not.
That's what makes it ungrammatical.
The to can be inserted either before or after not;
both of the following are correct: