I have faced the same problem and I've solved it by setting the Surface Emission property to a non-zero value. You can find it by going to the Physics tab of the flow emitter object, scrolling to the Fluid modifier section (here Type will be Flow), then finding the Settings > Flow Source > Surface Emission field).
All tutorials I've watched either skip over the property completely or say that zero means the volume of the fluid will be that of the flow emitter object. I have not found this to be the case, at least not in Blender 2.91.0 which is what I'm using. I couldn't get the flow to show up no matter what I tried until I set this value to 1 and then it all magically worked.