To communicate with your OPPO A7x in Fastboot Mode, you need to install the correct drivers on your computer. You can download the OPPO USB drivers from the official OPPO website or from a reputable third-party source.
fastboot mode...
Q: Can I access Fastboot Mode without a computer? A: No, you need a computer with a USB port and the correct drivers installed to access Fastboot Mode. How to Open Fastboot Mode on OPPO A7x