With the conquest complete, the tribes whose land is east of the Jordan River are able to return to their homes. Once they get there, they build an altar that causes a misunderstanding. The other tribes think the alter is to worship another god. The altar is actually supposed to remind them to continue to worship the God of Israel.
Misunderstandings happen, even in the worship of God. There are many differences between churches. Just because one church does things differently doesn’t mean they are wrong. That’s not to say we can worship God any way we want, but there is room for people to have different traditions than our own or find ways to draw closer to God that we might not find helpful.