There is a statement in the Book of Revelation that has been used to argue that nothing else can be added to Scripture. John wrote.

I warn everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to that person the plagues described in this book; if anyone takes away from the words of the book of this prophecy, God will take away that person's share in the tree of life and in the holy city, which are described in this book (Rev 22:18-19).

Does this statement mean the canon of Scripture is closed?

It Was Primarily Speaking of the Book of Revelation

First, we must note that this passage is primarily speaking of the Book of Revelation. John writes about the words of the prophecy of this book. This would be a specific reference to the Book of Revelation. John earlier had written that the entire Book of Revelation is a prophecy.

Blessed is the one who reads the words of this prophecy, and blessed are those who hear it and take to heart what is written in it, because the time is near (Rev 1:3).

He also used the phrase, "the prophecy of this book" two other times in the last chapter.

And behold, I am coming quickly. Blessed is he who heeds the words of the prophecy of this book (Rev 22:7).

John then said.

Then he told me, "Do not seal up the words of the prophecy of this book, because the time is near" (Rev 22:10).

In addition, when the Book of Revelation was written, there were still questions as to which books would constitute Holy Scripture. This issue had not been settled yet. There may have been New Testament books written after the Book of Revelation. For example, it has been argued that the Gospel of John was written after John wrote Revelation.

There Is An Implication for Rest of Scripture

While primarily speaking of the Book of Revelation these verses certainly imply that God has nothing more to reveal to humanity. Anyone who added after this time would be judged and anyone who took away from what was previously revealed. While this refers primarily to the contents of the Book of Revelation, Revelation does close out the program of God. Hence the application to the other books is possible.

The Ending of the Two Testaments Compared

The manner in which the two Testaments end is also significant. The Old Testament ends by looking forward to things to come.

He will turn the hearts of the fathers to their children, and the hearts of the children to their fathers; or else I will come and strike the land with a curse (Mal 4:6).

There is expectation of another prophet who is to come. However this is not the case in the New Testament. No new revelation or prophet is expected. Instead we have the announcement that the Lord Himself will soon return. This certainly gives the impression that nothing more needs to be revealed.

Summary

While Rev 22:18-19 does not explicitly state that God's revelation to humanity is complete it does imply that this is the case. After this time, any person who added to God's Word would be harshly judged. The same would hold true for any person who took away from what was previously revealed. Although these verses refer primarily to the contents of the Book of Revelation, it is possible to apply the warnings to anyone who would add or subtract from the entire Scripture.