This error occurs in two cases:
1. Your API key is outdated. Go to Google API console, create a new public API key there and paste it to the "API Key For Your Firebase Dynamic Links Account" field on the plugin's settings page.
2. Your API key is newly created. In this case, just wait a few minutes.
This article related to Shortlink by BestWebSoft plugin
Additional resources: Shortlink - User Guide
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