Regarding the issue everything seems fine, even though you have the right dimensions there can be many issues that cause this problem. Looking at the template size the size, looks about right. The shirt does have the correct dimensions, the shirt itself could be caused other problems.
Have you considered another attempt to upload the shirt again. The error message can be a false mishap or other errors causing the shirt to not upload. There are other threads, about this issue but it’s related to not having the proper dimensions. See here: