This release is compatible with Zotero 7 and Zotero 7 beta. The last version of BBT that is compatible with Zotero 6 can be found at https://github.com/retorquere/zotero-better-bibtex/releases/tag/v6.7.267. Zotero 6 support is in maintenance mode.
If you use Firefox, make sure to right-click and save the XPI file, not just clicking it.
You can verify you are on the latest version by going into 'Tools' -> 'Addons' in Zotero/Juris-M, clicking the gear icon, and selecting 'Check for updates'