Hello Jessica M,
I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this!
Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution in case you'd like to "Accept " the answer. Accepted answers show up at the top, resulting in improved discoverability for others.
Issue:
Could not successfully render Azure Video Indexer project.
Error Message:
Your project couldn't be rendered. We experienced a problem when rendering your project.
Solution: The customer @Jessica M renamed the files so that the file extensions were in lower case (e.g., "mp4" instead of "MP4" and "mov" instead of "MOV"). After doing this and re-uploading the files to AVI, there is successful rendering of the projects with subsets of those videos.