According to Moses, God told the people not to go to certain places that belonged to the children of Ammon. This was because God had commanded them not to go there, and they were to listen and obey Him.
Deuteronomy 2:37: Only unto the land of the children of Ammon thou camest not, [nor] unto any place of the river Jabbok, nor unto the cities in the mountains, nor unto whatsoever the LORD our God forbad us.
These verses state that the Lord gave all the land to Israel and that they possessed it, while Deuteronomy 2:37 specifies regions that were not taken, indicating a limitation at that time.
Joshua 21:43-45: And the LORD gave unto Israel all the land which he sware to give unto their fathers; and they possessed it, and dwelt therein.