Hello, I have DFRduino uno v4 and I have a strange error, namely access denied

You sure it is DFRduino Uno V4 or Arduino Uno R4?  Usually, you can plug and unplug the cable to solve the problem.  If not , try another usb port.  And if you are using Mac,  you may need to be careful about the Type-C to usb converter.  Some brand may cause uploading problem. 

pls help