I think you might be going around this the wrong way. Please keep in mind that my advice here is theoretical as I have no practical experience with the gear you're using.
The USB connection on the Tascam is probably designed to do one or both of these things:
1.) Transfer files between the Tascam and a computer.
2.) Work as an external sound card for a computer.
I highly doubt the USB connector on the Tascam is meant to be an audio input. However, the Tuscan does have a 1/8" stereo line-in jack that can be used as an audio input. That's your best bet for connecting the mixer to the recorder. You may have to experiment a bit but I'd suggest getting a cable like this:
https://www.bhphotovideo.com/c/product/1019924-REG/pearstone_smyc_2rm6_y_cable_stereo_mini_m_to.htmlAnd connecting it from the tape-out jacks of the mixer into the line-in of the recorder. Then, when you're all done recording, you can transfer the file(s) from the recorder via USB to a computer for further processing.