I know next to nothing about Canva; I’ve never used it for the reasons HotButton already mentioned.
However, the aspect ratio of 72"x36" is the same as 96"x48", which means, as HotButton also suggests, the printer could scale it up.
That said, scaling it up will cause the resolution of any raster (bitmapped) images in your file to decrease proportionately, which might or might not be a problem depending on the resolution you need. Do you know the resolution of any raster imagery in the original file? Does Canva provide information on those kinds of things?
On the other hand, if the imagery in the file is all vector data, you won’t need to concern yourself with it since vector data is scalable without losing resolution.
However, I’ve heard some nasty things about Canva-produced files from printers, so, for all I know, something will go wrong that wouldn’t go wrong in a file produced using more conventional tools. In addition, I’m assuming you’re working with a local printer who you can talk to about this kind of thing. If you’re shipping it off to VistaPrint or another online printer, whether or not they let you define output scaling, I have no idea.