Problem on first layer with Alfawise U30

Hello, I am a beginner in 3d printing with an Alfawise U30. The printer is assembled and seems to work. I have installed Cura to have Gcode files from stl files with the Alfawise U30 profile. I tried to print little objects but I have always the same problem : the first layer is bad, no details appeared. So, to have a reference, I got 3dBenchy stl file and generated a Gcode file with Cura. And like the other tests, the first layer is bad. And then, I wondered if maybe the problem can come from the way the way Gcode is generated. So, I made a test with the 3dBenchy Gcode file delivered with the Alfawise U30. But I had again the same problem : the first layer is bad. I don't know where is the problem and I don't have much experience, so I want to ask you if you can help me with this. Thanks