1. The object "roof_slope_v1accessory.gsm" needs another object (macro) called "m_slope_arrow.gsm".
Usualy it is placed in a subfolder named "Macro" of the folder where the object "roof_slope_v1accessory.gsm" is.
Verify that "m_slope_arrow.gsm" is loaded.
2. If "roof_slope_v1accessory.gsm" is pretty old there may be some incompatibility in the GDL code with the GDL interpreter in AC16 ?