An error message 'Boundary condition has not been defined' appears while running the model. Despite the fixed supports being defined at the base of the abutments and integrated into the construction stages, this error message persists. What steps can be taken to resolve this issue?

The boundary group fixed wasn't included in boundary change assignment and thus, the structure had no defined support for moving load analysis.  This was the cause of the error.

Besides this, it is suggested that first stage needs to be modified.  It was noted that elements were activated with 0 age in first stage and the loads were also activated on the first day.  This would cause huge deformations as the concrete age is 0 when loads start acting.  To go around this, I'd recommend giving the age as say 29 days and the duration of the stage as 1 day, as shown in the image below.

The modified model is attached for review.