That normally happens when you try to modify a field that is a "Calculation" type.
In you Relationship tab did you check "Allow creation of records"?
There is also a "prohibit modification" setting in field options that can trip this error for a specific field.
I think Jim's on the right track. What relationship did you set up for this portal? What kind of match fields?
Both Allow creation and Allow Deletion are checked. Prohibit modification is unchecked.
It's just a date field (not a calculation) but I'd like to be able to enter info into other fields in the portal too..
But what fields are used as match fields in the relationship? On the portal side of the relationship, is the match field a calculation field or a field that prohibits modification?
Once last rare possiblity....
When you go to your layout and in Layout mode....
Make sure all of the portal fields are using the right Table of Occurance, same as the Portal does. Many times we get 2 TO's and I have accidenty mixted them up.
Also make sure that all the field boundaries are inside the bounds of the portal row.
There may be a "hidden" field under the portal row. Move the portal set up down to see if a "hidden" field is underneath it.