John 1:18 says that "No man has seen God at any time." So how was it that Moses knew God "face to face"? (Deut. 34:10)
Moses, though he never literally saw the very person of God, he had a more direct, constant, intimate relationship with God than did any prophet prior to Jesus Christ.
Because God said: “Mouth to mouth I speak to him,” this reveals that Moses had a personal audience with God (by means of angels, who have access to the very presence of God; Mt 18:10). As Israel’s mediator, he enjoyed a virtually continuous two-way conversational communication arrangement. He was able at any time to present problems of national importance and to receive God’s answer. The later prophets simply continued to build on the foundation that had been laid through Moses.
The manner in which God dealt with Moses was so impressive that it was as if Moses actually had beheld God with his own eyes, instead of merely having a mental vision or a dream in which he heard God speak, which was the usual way in which God communicated with his prophets. God’s dealings with Moses were so real that Moses reacted as if he had seen “the One who is invisible.” (Heb 11:27)
For much more, see:
Face (Insight-1 pp. 801-802; Watchtower Online Library)
A Prophet Jehovah Knew “Face to Face.” (Insight-2 pp. 434-441; Watchtower Online Library)
No Man Has Seen the Father (Search For Bible Truths)