LastSubmit contains the details of the last record that you submitted through your form.
If you want to send the last submitted details to your Flow, the OnSuccess property is the correct place to call your Flow.
I suspect the reason why you receive this error is because 'Content approval Status' is a choice field.
In your call to SalesDealApprovalFlow.Run, if you type a '.' symbol after 'Content approval Status' , do you see any additional properties, such as 'Content approval Status' .Value? If so, I would change your call to include the property identifier that you see.