I tried the same but not with Postman but with a different application outside civicrm - I managed to create a contact without any error message

So I guess you are supplying the argument incorrectly in the POST. Please investigate (you need to send url and params separately).

I guess this helps !!!

I tried the same but not with Postman but with a different application outside civicrm - I managed to create a contact without any error message

so, Please try as below this should work

http://www.mysite.com/wp-content/plugins/civicrm/civicrm/extern/rest.php?entity=Contact&action=create&api_key=(myAPIkey)&key=(mySiteKey)&json=1&contact_type='Individual'&first_name='Tester'&last_name='Yolanda'

I guess this helps !!!