When, after clicking on 'complete installation', you receive a '500' error, please make sure to check the following document:
https://docs.shopware.com/en/shopware-6-en/tutorials-and-faq/notes-to-the-APP-URL
Also, please make sure that the URL with which you are trying to connect to Spreadconnect is reachable at all times. A '500' error means we cannot connect to your store's URL.
In case of addtional questions, please reach out to the Shopware support.