Hi Günce,
There must be no "data" directory on the target server, which the pg_basebackup command is run on, (this is a precaution set by postgreSQL to force admins to make sure no data will be overwritten) and the owner of parent directory of this "data" director must also be postgres user and postgres user must have necessary permissions on the parent directory. So, I wouldn't suggest using "/data" if it is right below the "/" (root) directory if I understand correctly.
Issue might be caused because of the above situation. Just my two cents..
Gokhan.