I had this same issue, and it turns out I needed to enable the Transaction Services API at the RVC level in Simphony EMC. Their documentation is here: https://docs.oracle.com/en/industries/food-beverage/simphony/19.8/simcg/t_sts_gen2_enable_option.htm
In summary:
Select the revenue center, click Setup, and then click RVC Parameters.
In the Parameters window, click Options.
In the Options section, select 74 - Enable Simphony Transaction Services Gen 2.
Click Save.
It took about 10 minutes before my calls started working after making this adjustment.