How to 'GetData' from an xsd:string
Wait, I found it. I needed to go up a higher level to the Test Case rather than the Test Step. It worked the first time I did it, however, it seems that SampleXML and GUID Custom Test Case Property values are being retained.....
So you are actually performing an incorrect selection which is causing this issue. At your Test Case Level (Your Test Case is DeleteUserAPIKey), there will be a Property Transfer Test Step that you have named SampleXML. So what is happening, is that every time, you are running the test case, your Property Transfer Step, is retrieving data from itself (The property transfer step has a guid that is not changing) and not the response that has the new guid. Hence why the new guid is not being retrieved.
don't have the selection of SampleXML in the Property drop-menu
Note that in my sample, I named a Custom Test Case Property SampleXML to play around with the data you provided as I dont have the same request/response in my system. In your sample you have named a Property Transfer Test Step SampleXML. This is 2 different things. I hope you have this clear.
If you have a look at point 3 in the Transfer properties page (https://www.soapui.org/docs/functional-testing/properties/transferring-properties) you will see that you would need to select the supplied response from the test step
So looking at your screenshot
In Step 2, the Property Transfer step that you named SampleXML
Source is your the request step AddUserAPIKey (correct)
Property should be Response (because you receive the guid in the Response sent back to you by the service you are invoking with your request in Step 'AddUserApiKey'...you would not see SampleXML)
once you select Response, the far right icon to select the field should now show the right data.