I bumped into a similar problem, and the cause seemes to be declaring bean fields as protected.
the message is at bit misunderstanding.
it just means that your cmp entity bean does not have a public property with the name you specified in the deployment descriptor. ( cmp-field / field-name tag )
the property ( meaning the field ) is accessible either if it is public or if it has a public getter method.
the error message relates to the last one.
but as i already set, the error could also occur, if you mistyped the field name, or didn't make it public in case you have no particular getter method.