No manifest.json was found at the root of the extension. The package file must be a ZIP of the extension's files themselves, not of the containing directory. See: https://mzl.la/2r2McKv for more on packaging.
Nulle manifest.json ha essite trovate al radice del extension. Le file pacchetto debe esser un ZIP del mesme files del extension, non del directorio que los contine. Vide: https://mzl.la/2r2McKv pro altero re le pacchettos.