Hi, I moved all the tested files from xampp to real server. It shows this error:

How to solve this?

Seems to be a CHMOD issue,
try to loosen CHMOD on admin/uploads/tmp folder through FTP.
This topic should be useful: viewtopic.php?f=8&t=7315

ok, I set the permission to 777, but will this changes cause any problem? I not quite understand about the permission, I only follow the instruction to do it.

Set the 777 permission did make things worked. But will it causing any other problems?

No, it will not cause any problems, it's just a usual operation.
777 permissions allow Owner, Group and Public to read, write and execute this folder.
These permissions are used for cache, logs and temp folders in every dynamic website, if it has such features.