|Jira Agile 6.3
Scope change warning is thrown for Issues when the sprint is started for not having values for 'Estimate' field & 'Remaining' field.
As mentioned in the error the issues do not have values from Estimate & Remaining field populated. These warning can be ignored and primary shown because the board is configured at Boards > Configure > Estimation as below :
Estimation Statistic : Original Time Estimate
Time Tracking : Remaining Estimate and Time Spent (Track time against issues using JIRA's Remaining Estimate and Time Spent fields. The Burndown Chart will be based on these two values.)