"Saved by an older Silhouette Studio"
Files saved by older versions of Silhouette Studio use an older layout that starts with silhouette04;. Cutpeek can't read that layout yet. If you have Silhouette Studio (the free edition is fine), open the file and save it again: current versions save in the newer layout, which Cutpeek reads.
"Newer than the format Cutpeek knows"
If Silhouette changes the format in an update, the file will carry a new marker. Cutpeek refuses to guess at a format it hasn't seen, because a wrong guess could give you wrong sizes. Until it is supported, export from Silhouette Studio itself.
"This file is cut short"
The file says how long its parts should be, and this one ends early. That almost always means a download or copy that didn't finish, or a file rescued from a damaged card. Download or copy it again. If you only have this copy, Silhouette Studio may not open it either.
"This is a ZIP archive"
Design Store orders and most design bundles download as ZIP files. Unzip it first (double-click on a Mac, or right-click and Extract All on Windows), then open the .studio3 files inside. With the one-time unlock, batch conversion accepts many .studio3 files at once.
"Encrypted or scrambled": protected files
Some files are deliberately protected so they open only in Silhouette Studio, on the account that bought them. Their design data looks like random noise instead of outlines. Cutpeek does not try to unlock protected files: that would mean breaking a protection measure, which isn't something it will do. Open the design in Silhouette Studio instead.
Not every Design Store file is protected, and Cutpeek can only tell from what is in the file. If a bought design does open, check its licence before using a converted copy elsewhere: designs are usually licensed for your own use, not sold outright.
"Only holds pictures" or "no cut lines"
The file opened, but there are no outlines to convert. It may contain only a picture placed for print and cut, which needs tracing in your cutting software, or the page may be empty.
"Already an SVG" or "a PDF" or "a picture"
The file isn't a .studio3 file at all, whatever its name says. SVG and DXF files can go straight into most cutting software. Pictures need tracing.
Something else?
If Silhouette Studio opens a file that Cutpeek can't, the file may use a feature Cutpeek doesn't read yet. Converting the design in Silhouette Studio is the reliable route for that file.