Hi @Shaney,
How is your flow created? Could you share a screenshot of your flow?
From the error message, it seems that the item you are going to update doesn’t exist. What’s the value you have specified for the Id field of action Update item?
I have made the following flow. When an item is created in a list, Start an approval, then Update item in the same list.
In the action Update item, select dynamic content ID from the trigger for Id field, and dynamic content Response for the Choice value field.
Please check the following screenshot for a reference.

Best regards,
Mabel