What error is displayed ?
The reason I ask I have been trying to submit a file for analysis from the virus chest and I got an error message. “! Something went wrong. Please try again later.”
If this is the same as yours it could be an issue with all submissions.
The submission https://www.avast.com/false-positive-file-form.php might work, but I just tried submission from the virus chest (Avast 19.5.2378 and I still get the “! Something went wrong. Please try again later.” message.