As Dennis Kho said, when you copy from SoapUI, make sure to copy only the content of the body.
Also, make sure that you have the namespace of that element correct.
You all were correct with the content of the body being required. I found another question that had one of the errors I had received. It seems that my SOAP request is using a custom namespace, so the custom request body in Mendix had to be formatted differently than SOAPUI. Here is a screenshot of my SOAPUI and then translated (working) custom mendix request body.
Note that I am using the "getCall" operation and the custom namespace happens to be "pub".
Here is the working SOAPUI (with dummy data)
Here is the working custom request template. Notice the declared namespace and the first line of code before the contents of the body. (with dummy data)