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Certain fields are never editable directly. These include the five built-in fields, all formula fields, and all lookup fields. In addition fields that were at one time reference fields can behave in such a way that they don't display either a read-only or an editable value. Here is the situation that creates these former uneditable reference fields.
You have two tables in a master-detail relationship. You delete the master table without first deleting the relationship. Then the reference field in the detail table still thinks its a reference field. So if you put this former reference field on an edit form you can't get it to display as an editable control. QuickBase is trying to display the field using the record picker. But the master table doesn't exist so it can't build a record picker so it displays nothing at all.
To workaround this attempt to delete the reference field. When you try this, QuickBase decides that it won't let you because the field is part of a relationship. It then offers you a link to edit the relationship. If you click on the link you see a half existent relationship. Click on the delete relationship button. Accept the confirmation dialogs. This will remove the pointer in the reference field pointing to the nonexistent master table. Now the reference field is no longer treated like a reference field and will display on edit forms normally.