Are you using USB Full Speed, or High Speed?
Full speed is little more than CMOS logic level, and doesn't care at all about traces this short. High speed is more critical, but won't mind a poorly matched impedance for trace lengths up to 200mm or so.
Controlled impedance is impractical on 1.6mm two layer board. If you must, consider reducing board thickness to 1 or 0.8mm or even less (this reduces trace width below 0.5mm, still rather inconvenient), or positioning the receiver even closer to the connector, or using 4-layer construction.
Note that Altium will not calculate the impedance without a reference plane. It doesn't know to check copper underneath or around a trace. Instead, it assumes a signal layer is empty, and a plane layer is solid copper. Better to calculate the trace width using an external calculator tool, and set design rules based on that.
BTW, note that the USB common mode impedance is also specified. This sets how close the traces should be to each other, as well as their width, and the distance to nearby ground fill. Again, you don't have much option here, so follow one of the above strategies.
Tim