Here’s how Apple describes inconclusive ECG results on its support page:
An inconclusive result means the recording can’t be classified. This can happen for many reasons, such as not resting your arms on a table during a recording, or wearing your Apple Watch too loose. Learn how to get the best results.
There are other factors that can cause inconclusive Apple Watch ECG results as well, including an elevated heart rate, pacemaker, and more.
More Info: 9to5mac.com