Hi Ra,
Moodle will tell you that you need Canvas, but it shouldn't be stopping you from carrying on with the installation because canvas should already be in your themes folder as part of the default setup (base and canvas were retained when overlay and other themes were moved out to the plugins database for exactly this reason).
Check the plugins installation page again and see if you are able to carry on with the installation.
If not and the canvas theme is missing (has been accidentally deleted or something like that), try redownloading moodle itself, extract the zip file on your computer somewhere and copy the canvas theme folder over into your moodle/theme folder.
Richard