Is there a single word or phrase for "inability to say no at a behavioural level"

I think people-pleaser might come close to what you are looking for; although, the way I have heard it, that term generally implies a desire to please people so much that it becomes harmful to their own wants and needs, due to being so people-pleasing.

So, these folks (people-pleasing people?) have a hard time saying 'no' due to their desire to please others. However, your example might well fall outside this: whlie the person habitually does say 'no', the example given (with the candy) could also be explained by a learned reluctance to say 'no' (or some other dislike of the word) rather than a desire to please.

For those that do not fit the people-pleasing description; you might say such a person is not inclined to say 'no', or has a disinclination to saying 'no'.

For those that really hate the word, they might be averse to the word 'no'; or, perhaps, simply 'no'-averse.

If the inability to say no implied acceptance, then I'd say pushover.

Since you seem to be saying, rather, an inability to say no directly but making acceptance so unbearably complicated as to constitute an effective no, I'd say passive-aggressive.

What you're describing does not sound like a personality trait — after all, the receiver was trying to reject the gift, rather than just silently but unwillingly accepting it. Rather, what we have here is a difference in communication styles, and this is an example of indirectness. The receiver, in his/her mind, was saying "no", and the giver just wasn't taking the hint.

"Indirectness may be reflected in routines for offering and refusing or accepting gifts or food, for instance. . . . Visitors from the Middle East and Asia have reported going hungry in England and the United States because of a misunderstanding of this message; when offered food, many have politely refused rather than accept directly, and it was not offered again."
(Muriel Saville-Troike, The Ethnography of Communication: An Introduction. Wiley, 2008)

I suggest yeasayer:

a person who habitually agrees with or is submissive to others

I'm going to say polite refusal. It can be present as a social skill, as a say-no characteristic or as incorporated in a culture.

As a culture, Japanese people are the master of "saying no without saying no".

For example, Japanese word iie, which translates to "no", is rarely used. They use the word chigaimasu, which is a polite way of saying no, and translates to "different" (or "wrong" in some contexts.)

In general, this kind of indirect people can seem as a waffler also because of the vagueness and indecisiveness in their speech and their understatement can be explained as an unobtrusive behavior. This behavior can show itself as both a polite hesitation for a desired offer and a polite refusal of an undesired offer.