Curve length is too small for Revit’s tolerance….Parameter name Endpoints
It has to do with the wall the frieze needs to reference. Since the portion of the wall she is trying to select for the frieze was created by editing the wall profile, there is no base constraint like there is in the shaded area.
It is possible our tool does not recognize that as an actual wall.
To get around that; using the wall grips, temporarily drag the 1st floor wall (red arrow) so that area of the gable was represented by the wall baseline:
Add the frieze; then manually moved the wall baseline back to its original location.