It appears in one of Salesforce's recent security update, it prevents iframes such as Tableau from being presented within Salesforce. Your next best option is to present the dashboard as a Web Tab. Downside is the data loses the data interaction and connectivity Canvas had.
You can find more information on Salesforce's view on iframes below:
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HI Bob - At Senturus, we have a working demo environment we are happy to show you, and can implement this for you at a reasonable cost. Having Tableau data - which allows you to incorporate SFDC AND NON-SFDC data into Salesforce, while maintaining context and allowing users to remain in their UI of choice is a very powerful business tool to optimize rep productivity and deal conversion.
Please reach out if you'd like to schedule some time to talk. Thanks - Michael (email@example.com)