Why specific file directory fails to redirect but its parent folder succeeds?

After trying a million of ways, I contacted the super admin and asked for permission to log into WP Engine, our host. I found there is a Multisite setting too. So I clicked “convert to multisite”, and, bang! the error is resolved. Subsites can work properly now.

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Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)