I query the PropertyAssociation of the rectangular bar magnet this way:

The magnetic flux density already looks bad. I think the \[RuleDelayed] for the M vector needs a closing curly brace "}". Addition to that, when I Activate it, it activates only the Subscript["M",0] QuantityVariable, but does nothing for the for the \[RuleDelayed] to calculate the H vector. Also a //. is needed before the xm, ... etc variables instead of the "," and those variables should be in curly braces.

mfd = Entity["PhysicalSystem", "RectangularBarMagnet"]["MagneticFluxDensity"]

After activation and a substitution: