I'm trying to print a 40 column receipt to my newly installed Epson receipt printer but it is not showing in the dropdown list in the "Destination" field in MicroBiz; what could be the issue here?

Click "Manage" at the bottom of the dropdown list. This will open up the Windows Printers and scanners screen. Check to see if it is visible there. If not, that means that the driver needs to be installed.

The driver for the Epson TM-20II printer with a USB connection is the advanced printer driver v5.09 and may be found on epson.com or directly downloaded from this DropBox link