It must be that you have not connected your mobile phone or p3 to the computer.

First, the interface problem. There are two or three USB slots. Try changing them.

The second problem with mobile devices is that after plugging in the data cable, your phone will prompt you to select it.

There may also be a yellow prompt box that pops up after connecting and playing the file. Whatever, please choose carefully. That's basically it. Otherwise, your mobile device is broken.