Either you've exhausted your file transfer limit for the month, file transfer is disabled for your server, or there is a problem on the hosters end.
Note that not all hosters necessarily include file transfer in your package, so if there is no file transfer included you'll have to see if there is a filetransfer option you can book in addition to your server.
You can check the quota by editing the virtual server (right click server name -> edit...) then click More, select the Transfers tab and check the upload quota number. If it's above zero then close the edit dialog, right click the server name again and select Connection Info. The last row has File transfer bytes transferred. If the first number (the IN column) is greater than the quota minus the size of the file you're trying to upload then you've exceeded the allowed file transfer traffic this month.
Note that this can be the case even if the number in the server connection info is smaller, if the server has been restarted in the current month.
It's also possible that your group has a lower limit set. Check your permission overview and search for upload_quota. That is the amount of megabytes you're allowed to upload each month. Of course if the server limit is lower than the server limit counts.
If the group limit is lower than the server limit you may be able to increase the group limit in the permissions, but other than that you'll have to contact your server hoster to fix it.
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