As I understand you choose to upgrade to Stylus 4 version which is optional for Stylus 3 users. 

For users who do not want to go with Stylus 4 (new subscription license) we recommend not to upgrade, or if you made upgrade - we recommend to use function "revert to previous version" which brings you back to Stylus 3 and old license you can freely use.

If you are unable to find or use function "revert to previous version" and do not want to upgrade to Stylus 4, we recommend you to make repair of Stylus 3 installation. This is done as you download Stylus 3 version image, load it to USB stick and then boot it from USB stick, as per instructions given on our web page:

Get V3 image: 

https://audiokernel.com/downloadimage3.php

Install V3 image to USB stick (same as V4 image, but use downloaded image given above:  

https://euphony-audio.com/v4/installation/  

After booting USB, register with your V3 voucher code, and make install to your HDD. You can choose "repair" installation so you do not loose your library.

Attention: You need to install to your old hardware where you installed Stylus 3.

Here is info about voluntary upgrade to Stlyus 4  for Stylus 3 users. 

https://euphony-audio.com/v4/promo-v4-for-v3-users/