Hi
I have trouble using Arcgis 2.9.9 when I try to change simbology. Also, I tried to update but it is not possible
There might be a symbology bug at 2.9.x, so try upgrading to 3.x as there are many 3.x versions available.
I would upgrade from ArcGIS Pro 2.9.2 to ArcGIS 3.1.x using the workflow here and see if that resolves your symbology issues.