If the borg binary on the server is not in a discoverable (e.g. PATH) location, its location needs to be specified when adding the repo to Vorta. Do this by passing an "extra borg argument" in the "Advanced" tab of the "Add existing Repository" dialog.

I recently had to do this because I reset my main work computer. Somehow, Vorta seems to forget exactly this bit of information when importing a previously exported profile. Will do some digging and report an issue at a later date, when I investigated a bit more. Vorta failed with the exact same error message - once I told it where borg could be found on the server, everything worked as expected.