Hi Rhian,
Can you provide some screenshots of your process? The message is suggesting you are trying to complete an action that requires ownership eg., an automation must take possession of an item to make changes to it, this avoids unwanted changes that could have an impact.
The automation will need an uninterrupted flow from "Get Next Item" to "Mark as Completed", and you need to always have called "Get Next Item" before you "Mark as Completed" again, otherwise Blue Prism still has the same old data there. You can validate this by reviewing the [Item ID] text item you get from "Get Next Item" and ensuring this value is new and different every time you use "Mark as Completed".
Hope this helps,
James