Hello @ErikF04 ,
The insert Canva option is only available through emails built using the cross-device / cross-platform editor (typically referred to by us as CPE), or for landing pages. If you're not seeing the option, your template may still be on the 3GE email builder. Eventually all templates in 3GE will be fully, seamlessly converted over to CPE.
In the meantime, the easiest workaround would be to utilize the separate Canva button available in the Library tab of the top navigation bar. If for some reason this button isn't available in your Library or doesn't work, you should also be able to export the image from Canva as a JPG or PNG, then upload it like any other image into the Library.