If you edit the filter used on the board and don't use the ORDER BY Rank, the "Add Sprint" button on the Plan mode is not enabled.

Using the below query the "Start Sprint" button is not enabled:

Project = Test

Once you edit the filter and include the Rank field, the "Add Sprint" button is displayed again.

Using the below query the "Start Sprint" button is enabled:

Project = Test ORDER BY Rank ASC