There can be a two possible causes for this problem:
You have connected to our signaling serves but have problems with establishing a connection between locations. The first think to check is if the required ports are open. The second step would be to try switching to the WebRTC protocol.
The Receiver can't decode the stream from VT Publisher or VT Publisher can't encode the stream with the selected codec. In case, of old GPU hardware or out-of-date GPU drivers, some machines can have limitations for certain transmission methods (especially for "SRT-HEVC"). Check your hardware configuration and switch to "SRT-AVC" or "WebRTC" transmission modes if H.265 (HEVC) is not supported. If H.265 is support try testing with WebRTC first, update your GPU drivers and test again with "SRT-HEVC".