Request Form Example doesnt work

Hello!

Im new in Appian so im trying to get it all little by little. By the moment, im stuck trying to make work this form example:

By default, if i only pick that example and save the form, when i go to the task, i get this error message when i change the value of any field:

If i TEST the form, it works. I think it works because purchaseRequest have an input:

while in the interface parameters, that variable is empty:

I tried to put the same structure....but it doesnt work, same error.

So, after sereral hours finding a solution, i have to give up and asking for help.

Can anyone help me?

Thank you so much :)