Please try the suggestions below to check whether the issue can be fixed:
Ⅰ. Please confirm that the source files can be played normally in Spotify.
Please open Spotify and check whether the songs you’d like to convert can be played normally in Spotify.
Ⅱ. Should you encounter any of the situations as described below, please try the detailed steps to check whether the issue can be fixed:
 The songs play for about 20 seconds in Spotify and then skip to the next song for converting and end up with “conversion failure”.
Step 1. Please check whether the system is connected to any audio output device. If it is, please disconnect the external audio device (including the audio devices connected by Bluetooth, like speakers or headphones).
Step 2. Restart your computer and convert the songs again.
 Conversion progress for a single song performs normally but fails when it is almost completed.
Solution: Reset Documents Permissions.
For macOS10.15 users, please check System Files-> Security & Privacy to see if Files and Folders has been granted access to the relevant Documents directory. If it’s not found here, please perform the reset operation.
Search “Terminal” in the Finder and open it up. In the Terminal application, execute the following command to reset the access permissions of Documents.
tccutil reset SystemPolicyDocumentsFolder "com.audifab.spotifyconverter"
There will be a pop-up window to request permissions when AudiFab is launched next time.
 For other situations, please go to Help in the Menu bar and select Uninstall the Recordinig driver, and then reboot your Mac for a try.