The RETS error message means that we were unable to communicate with your MLS.
This is normally caused by 2 things:
Temporary failure communicating with your MLS (meaning that you can probably re-fetch the listings in a few minutes).
Or, Invalid MLS Credentials (be sure to verify your MLS ID and Password)
You can verify your entered MLS credentials by navigating to Settings > MLS. There, you can edit your MLS credentials accordingly.