In your first message, you mentioned that the storage disk at the on-premise server is approx 2TB. However, as a free trial account, your disk requirement is above the Quota you are allocated which you already identified from the
Quota page of the Cloud Console. There is a limitation of Quota when you are enjoying free trial. You can find the details at this
link. Unfortunately, you will not be able to request for additional Quota as long as you upgrade your free trial account to paid account. As you mentioned less than 5% of your disk is currently in use at your on-premise server thus I would recommend using smaller disk size which falls within the Quota and then try the migration again.