, a る-Verb meaning ‘to spread across’ (usually 2 different activities or thought processes), often follows the ます-stem of other verbs in order to highlight that ‘(A) is not possible’. However, 兼（か）ねる
often indicates that the speaker would actually like to do (A), but cannot due to some external circumstances such as difficulty, already being busy, being hesitant, etc.
This particular grammar pattern will most frequently be seen in its non-kanji form, although both are possible.
Even in situations like this we can’t give you a refund.
That is information regarding our client’s privacy so we cannot answer that.
We cannot accept reservations by phone.
is somewhat formal, and may be used in cases where a phrase like できない would be considered too strong, so the speaker wants to lightly imply that (A) cannot be done.
Fun-fact - As 兼（か）ねる
means ‘to spread across’, this expression can be thought of as similar to ‘to be a stretch’ in English. It just indicates that doing the action of (A) would be a stretch of the speaker’s ability or capacity.
It would be a bit of a stretch for us to accept exterior work, as we specialize in interiors.