b = sum(row[0] for row in arcpy.da.SearchCursor(fc, 'QUANTITY_SOLID'))

with arcpy.da.UpdateCursor(fc, ['QUANTITY_SOLID_SUM']) as cursor:
    for row in cursor:

row[0] = b
    cursor.updateRow(row)

import arcpy

# Define the feature class
fc = r'C:pathtoyourfc'

# Use a generator expression to populate a list from the 'QUANTITY_SOLID' field
b = sum(row[0] for row in arcpy.da.SearchCursor(fc, 'QUANTITY_SOLID'))

with arcpy.da.UpdateCursor(fc, ['QUANTITY_SOLID_SUM']) as cursor:
    for row in cursor:
        row[0] = b
        cursor.updateRow(row)