That’s exactly right. It’s a simple check to determine if your device is up to the task of pushing enough pixels to two screens simultaneously. Here’s a case where we conclusively determined a cause of blank screenshots was caused by an underpowered laptop.
A lot of people on this thread have been trying to help you, and we’re all rooting for you. I get that you’ve been frustrated for a long time about this, but if you’re not willing to try the suggestions, we’re not going to make progress.
Will you disconnect the HDMI cable (or DV, or whatever) from your laptop and try a brief ride of greater than 2km? This is the minimum ride distance for bike sessions to upload to Zwift’s servers.