Hi,
since the used syntax seems to be correct, I assume there is somehow no value available for your attribute.
Could you please make sure that your attribute "co" is actually defined and has a value? You could leave the "attributes" section out and print attribute names and values for all user attributes:
<fsp:userAttributes>
Attributname: <%= attributename %>
Attributwert: <%= attributevalue %>
</fsp:userAttributes>
as stated in the official Dynamic Personalization documentation on page 65. Your attribute's name should be printed. Does that work?
Best regards,
Hannes