Before creating the wave, if you are able to generate a runbook template CSV file, and edit it (such as adding the target instance type), it might be that the tool is not recognizing the formatting. Have you tried editing the downloaded file in Google spreadsheets instead, and saving it as CSV? I found this Youtube video that might clarify some steps (Minute 20:45), and despite it is about migrating from AWS.
Another theory is that the migration tool does not have access to your local files. Depending on the OS, you may need to grant or modify permissions on the folder/csv file.
After checking the above, and if you are still having the same issue after selecting the file, I recommend creating an issuetracker report to investigate the tool behavior. Please provide the version you are using, and attach a screenshot of the error (redact personal information such as email/project ID). Also, please provide all the steps performed (for reproduction purposes) along with a description of the OS, and the steps taken to give permissions on the local machine (if applicable).