It might be a permission issue. It looks like the same error I get with RPGM MV games running in HTML in a freshly installed Firefox.
If you're in Firefox, changing a browser configuration might help. To do this, type about:config in the address bar and press enter. In the new page, search for "privacy.file_unique_origin" (without quotation marks) and set it to false.
If you're in Chrome, sorry, but I don't know if there's a similar config