I am stuck on very basic requirement. I have a person or group field on SharePoint Online list which I want to update from a SharePoint Designer workflow. When I assign the value "User who created current item" to this person/group field. However, it is not updating and giving me this error.
"The workflow could not update the item, possibly because one or more columns for the item require a different type of information."
My bet would be that the person or group field that you want to update allows for multiple selections. A properly formatted string for a single user in a multiple user lookup field is 00;#Name, Display. As such I would recommend creating a local variable, for example "created_user" and have the first step of the workflow be Set that variable to [%Current Item:Created By%];#[%Current Item: Created By%] where the first [%Current Item: Created By%] returns User ID Number, and the second one returns Display Name. Then when you do the update list item action when you get to the Value assignment screen, in the "To this value:" section use the fx button and NOT the "..." button. The Data Source will then be "Workflow variables and Parameters", the "Field from source: will be the variable that you created, the "Return Field as:" will be String
