You see this, or a related message, when a file is too large or of a type Studio does not take:
"Files can be up to 50 MB."
"Images can be up to 10 MB."
"Audio can be up to 25 MB."
"Upload an image (PNG, JPEG, WebP), a video (MP4, MOV, WebM) or audio (MP3, WAV, M4A)."
Limits
Images: PNG, JPEG or WebP, up to 10 MB.
Video: MP4, MOV or WebM, up to 50 MB.
Audio: MP3, WAV or M4A, up to 25 MB.
What to do
Compress large images, or save them as JPEG.
Trim or compress video before uploading, or upload a shorter part.
Export audio as MP3 for a much smaller file.
Full podcast episodes up to 2 GB go in Podcasts, not Assets.