If you like, post the code you used and we can help you debug it.
I just tested my logic and it worked in a file geodatabase. Here is updated code that is a little more flexible.
def main():
import arcpy
import os
temp_gdb = r"N:\TechTemp\BlakeT\Work\TEMP.gdb"
fc = os.path.join(temp_gdb, "TEMP_Line")
sortFieldName = "Note1"
seqFieldName = "SeqID"
# Step 1
existingFields = [field.name for field in arcpy.ListFields(fc)]
if seqFieldName not in existingFields:
arcpy.AddField_management(fc, seqFieldName, "SHORT")
# Step 2
cursorFields = [sortFieldName, seqFieldName]
sql_postfix = 'ORDER BY {}'.format(sortFieldName)
with arcpy.da.UpdateCursor(fc, cursorFields, sql_clause=(None, sql_postfix)) as u_cursor:
# Step 3
for seqid, row in enumerate(u_cursor, start=1):
# Step 4
row[1] = seqid
u_cursor.updateRow(row)
if __name__ == '__main__':
main()
