The issue was with your EntitySetName property, when calling the API in BC it does not use EntityName but instead it uses EntitySetName

e.g. {{baseurl}}/v2.0/{{tenantId}}/{{environment}}/api/<api publisher>/<api group>/v2.0/companies({{companyId}})/PunchoutReceiver

use this instead
{{baseurl}}/v2.0/{{tenantId}}/{{environment}}/api/<api publisher>/<api group>/v2.0/companies({{companyId}})/PunchoutReceivers