THE CAUSE FOUND!!!
I found the cause of this issue. So obvious. Everyone who’s having this issue please follow this instruction!!
DON’T REINSTALL THE WORDPRESS OR BUDDYPRESS. You should just follow the steps below.
1. go to siteurl/wp-admin
2. go to setting -> permalinks
3. Change common settings EXCEPT DEFAULT. If you set it as default, the issue occurs.
If this doesn’t work, please reply.
Thank you all!