ZahidUllah I fixed this for the next release. Your hosting environment doesn't allow saving to the temporary folder or it is not configured. You can edit the file mentioned and change line:
$mapTemp = tempnam(sys_get_temp_dir(), $mapFile);
to
$mapTemp = tempnam(storage_path('tmp'), $mapFile);
Let me know whether that solves it for you. Please note this edit will be reset whenever you update flarum/core (for beta 9 this is not an issue though).