Payments API error

Hello,

I'm getting the following error in my code and in the Payments SKY API (https://developer.sky.blackbaud.com/docs/services/payments/operations/CreateCheckoutTransaction/console)

message: 'Transaction is required.',
error_name: 'ChargeTransactionNullArgument',
error_code: 14,
raw_message: 'Transaction is required.'

What is this Transaction and how do I pass it to my request?

Thank you!