The file is too large. Files are accepted up to 2 GB. Export a lighter version from your phone or editing software, or cut it into parts.
The video is too long for my plan. Each tool has a longest video per plan: see Accepted files, sizes and lengths. Cut the part you need with Trim, or see Plans and prices.
The format is not accepted. Convert the file to MP4 first with the converter. Each tool article lists the formats it accepts.
The video is too short. Some tools need a minimum length (for example AI Clips).
A free account is asked before the upload. Some tools need an account from the first use: see Using Klipa without an account.
The upload stops. Keep the page open, use a stable connection and try again.