Check your datatable variable. Looks like Type column is not present or maybe a typo/contains space.
Check once whether you selected Add Headers option or not.
i don’t have Add Headers
Easiest way to see the mismatch is run your code in debug with the breakpoint on this assign.
Once it breaks see your Locals panel to what is contained in the TransactionData variable.
Looks like the variable could be empty/not assigned correctly.
Add a screenshot of the value to help further.
Mapping looks good. Can you add a breakpoint on the assign activity and run your process in debug mode?
This way we can get the values that has been assigned to the variables at runtime.