If I understand you correctly, you plan on using rubrics to assign the grades for an assignment. For that rubric, you would prefer that the when a criteria rating is picked that it be A, B, C, D or F. Then the overall grade would be based of all the criteria scores and show as A,B,C,D or F.
Furthermore, for the entire class all assignment grades (as letters) are used to determine the overall course grade shown as A,B,C,D or F.
if that is what you want, that is not possible to do in Canvas. Canvas needs numbers in order to compute scores. numerical scores can then be converted to letter grades however you want to do that. it is probably possible to setup a course such that letter grades are used(visible), but numbers will have to be used in the background. Maybe someone has done something like this for a course already.
(following is a start of something that gets confusing quickly) However, You can somewhat simulate that effect in how you set up grading. You would need 5 ratings for each criteria with point values of 4,3,2,1 and 0 (representing A,B,C,D,F) Then for the assignment you would select display grade as a letter - and you will need a grading scheme for the assignment that will assign a letter based on the numerical score earned for the assignment.
I'll stop there, because it is getting confusing to me, and I would actually need to create an entire course setup to try it out with grading of various assignments and display of the grade values. Everyone would still see numerical values in places, but also the letter grades in places as well.