The app said the upload worked. Why is the car blank?

The car checks the file, not the app. A name over 30 characters or with punctuation, or a file over 1 MB, is dropped without a message.

Does the car need to be awake?

Yes. If the car was asleep during the upload, wake it and reopen Paint Shop.

Ten wraps. The eleventh does not show until one is removed.

The checklist for a wrap that uploaded fine but never appears in the car.

File name — only letters, digits, _, - and spaces; 30 characters at most. This is the most common cause.
File size — under 1 MB. Re-export with fewer photos or simpler layers.
Format — PNG, square, 512–1024 px.
Count — ten already in the car? Remove one.
Car awake — wake the car, then reopen Toybox → Paint Shop.
Software — very old car software predates custom wraps. Update.

Exports from this editor already pass the first three; if you renamed the file afterwards, that is where to look.

A checklist in order of likelihood is in the complete Toybox custom wrap guide.

The app said the upload worked. Why is the car blank?: The car checks the file, not the app. A name over 30 characters or with punctuation, or a file over 1 MB, is dropped without a message.
Does the car need to be awake?: Yes. If the car was asleep during the upload, wake it and reopen Paint Shop.
Is there a limit?: Ten wraps. The eleventh does not show until one is removed.