What browser you are using to access your website with the iframe? I have tried with my own Tableau Server using Google Chrome. I went into the console and I saw an error message "Refused to display 'http://localhost/' in a frame because it set 'X-Frame-Options' to 'SAMEORIGIN'. I believe this is because the clickjacking protection feature in Tableau Server. You can disable the clickjack protection by following the KB article below:
The top-level page from Tableau Server cannot be loaded in <iframe> elements. This includes the sign-in page. One consequence is that you cannot host Tableau Server pages in an application that you create.
Please make sure your tableau server accessable and try below code replace google with tableau url and some applications may not support full urls
try with tableau home page urls without special characters and then play around
I disabled the protection setting and it works now.