The release issue.
The current cover art is too blurry and would not be accepted by the stores. Please upload a new cover art with a higher resolution.
The stores have a high standard when it comes to cover art resolution. Even if your cover art looks quite sharp it could be too blurry.
Let's take this example:
Looks fine right?
Even though the cover looks quite sharp in this view it is too pixelated. Why? If we zoom into the cover we can see individual pixels of the cover.
How do I fix the issue?
First of all, make sure the cover is LARGE enough.
If you select a picture that is too small and stretch it to to make it at least 3,000 x 3,000 pixels (Minimum size that the stores accept) chances are high that it will be too blurry.
Once you have selected a cover zoom into it and check if you can recognise individual pixels. If you can then your cover is unfortunately too blurry and is not likely to get sent to the stores.
Here are also a couple of tools that could help: