I have been having trouble merging point features together in ArcGIS Pro due to the error "001156." I've been looking up solutions but they don't necessarily help me out! I don't know which output value to use either...ArcGIS Pro is fairly new to me!
Further to my comment, "your
assessment_notes field probably has a field length smaller than the piece of text you're trying to insert" -- if you click ERROR 001156 from the results area of the geoprocessing tool, it'll open the help to that error topic with more information. That should be the first thing to do when encountering a tool error.
A value could not be added to the output field; for example, text values cannot be added to numeric fields, and text values cannot be added to text fields if the values are longer than the field length
The solution referenced says to change the field type. In your case its probably just changing to a new field with a longer length.