Start/Stop Work button is not enabled nor available till an Estimate is entered for the given case by a user.
Default estimates are not available by design, because the expectation of the Evidence-Based Scheduling (EBS) algorithm is that giving an Estimate should be done by the person who will actually do the work (actually push the Start/Stop button).
As described in the Evidence-Based Scheduling (EBS) article, EBS needs an Original Estimate value to compare it with the Actual Time spent delivering the case. Based on this data (original estimate vs actual time) the algorithm is able to estimate overall delivery dates.
As described in the Estimate Value Used by EBS article, the Original Estimate will be considered the estimated value that is available in the Estimate field right before starting the work (first time clicking the Start Work button).
And whoever entered the estimate that existed at the time when the work started, will have the case reflected on their EBS and history.
This design gently pushes the users to have estimates entered by the person who will be actually doing the work, instead of having their managers or project managers entering default or wrong estimates.