An organization can nominate candidates in several way, including by a nominating committee, from the floor, or by ballot.

Most often, a nominating committee presents nominations, and the assembly has the opportunity to present additional nominations from the floor. The nominating process should not be confused with the election to office.
Robert's Rules of Order
states that a person does not have to be nominated to be elected to office. If the vote is taken by ballot, there is always the opportunity to write in a name. In this case, a person can win as a write-in candidate without ever being nominated.

I respectfully disagree with your characterization of "Robert's rule of accepting nominations from the floor after the nominating committee's report and immediately prior to elections." I find no such rule. I find only the passage I quoted in my first post, including the phrase "while the election is pending or earlier" (emphasis added).

I find nothing in RONR that requires nominations to be accepted immediately prior to the election; permission yes, requirement no. This PTO's bylaws explicitly include a cutoff date for the acceptance of nominations, and I find nothing in RONR providing any authority for overriding that cutoff date.

Obviously the point is moot now, though I'm curious to know from OP what happened.

If I had a dog in this fight I'd come at it from a different angle.

The portion of bylaws that Anonymous is sighting refers to duties of the Nominating Committee. It states that the committee is to accept nominations up until 10 days after the April meeting.

There is nothing in that statement that precludes Robert's rule of accepting nominations from the floor after the nominating committee's report and immediately prior to elections.

If all the committee's nominees were to drop out prior to the election, this PTO would have no candidates to present unless the PTO allows RONR to kick in and allow for nominations from the floor immediately prior to elections.

But since Anon's meeting has come and gone, it is too late to challenge the election no matter how you interpret their bylaws.

This PTO's bylaws are ambiguous and should be amending.

Another glitch I see is the specific wording ".....Committee shall also accept nominations for all offices from the membership...." This implies that the names accepted by the committee are already nominees- not people wanting to be picked as nominees by the committee. Therefore the committee must submit to the membership all the names brought to them. The purpose of a nominating committee is to bring forth the best candidate(s) for the job- not every person that wants the job. Otherwise, why have a committee?

Robert's Rules cannot override your bylaws. In fact, Robert's Rules itself says, "Except for the corporate charter in an incorporated society, the bylaws ... comprise the highest body of rules in societies as normally established today."

Furthermore, even if Robert's Rules could override your bylaws, there's nothing in there, to my knowledge, saying nominations must be accepted until the moment of the vote. In fact, there's this, in Section 45, in the description of nominations when there is a nominating committee: "After the nominating committee has presented its report and before voting for the different offices takes place, the chair must call for further nominations from the floor. ... In many organizations, nominations from the floor are called for immediately after the presentation of the nominating committee's report -- while the election is pending or earlier."

Ask them to cite the specific passage in Robert's Rules that gives them the authority to accept a nomination after the period specified in your bylaws has expired. (I'm assuming that period actually did expire before the new nomination was submitted; it's possible that it didn't. If your April meeting was on the 29th or 30th, then the 10 days would not have expired until one or two days ago. I assume that's not the case or you wouldn't have asked the question.)

I have a question about our PTO's membership vote tonight. At the April meeting the nominations were announced and the vote is tonight. All positions were running unopposed. A member was then added in 2 days ago to run for one of the previously unopposed positions. There is a question of the legitimacy of the nomination being accepted. The bylaws indicate that the nomination is too late, however, the PTO board has stated that they are following Robert's Rules of Order and allowing the nomination and any nominations up until the moment of the vote this evening.

I would like feedback and thoughts on how to interpret deadline for nominations given the bylaws listed below.

The PTO's bylaws state:
The Nominating Committee shall present the names of the nominees at the April Membership meeting. The Nominating Committee shall also accept nominations for all offices from the membership of the PTO in written form at the April meeting. Further nominations will be accepted for ten (10) days following the April meeting.

The Nominating Committee will review the nominations and will then contact each nominee for consent to candidacy. The Nominating Committee will prepare written ballot forms containing the names of the nominees for each office prior to the May membership meeting.