Oppo A71 2018 is oppo qualcomm device which implement VIP (Validated Image Programming) to their programmers (firehose). Devices with VIP Enabled only recieve limited command from their programmers, mostly only write command is allowed by calculating digest table to their official image firmware.
So, if you want to clear the user lock, especially for Oppo Qualcomm Devices, all you can do is just by full flash it by official firmware only.
And yes, current UFI Android ToolBox is supported for Oppo A71 2018.
Another UFiXers already make a video regarding to this subject, kindly check this out.