Your phone uses Bluetooth SPP (no driver). Your PC requires a USB driver. Use the USB cable and reinstall the driver.
Sites like DriverPack Solution or MajorGeeks sometimes host these drivers, but verify the checksum (file size). a9 thermal mini printer driver download
This is the most common mistake. Windows will attempt to install a generic driver that fails. Your phone uses Bluetooth SPP (no driver)
The keyword "driver" is slightly misleading. For mobile devices (Android/iOS), you rarely need a traditional .inf driver file. For Windows and Mac, however, a proper driver is non-negotiable. a9 thermal mini printer driver download
To download and install the drivers for an A9 thermal mini printer (most commonly the PeriPage A9