Welcome to the Reaper Forums!
Make sure to check out the User Guide (
https://www.reaper.fm/userguide.php) and the Videos section (
https://www.reaper.fm/videos.php) in general.
Is Input Monitoring enabled for the tracks in Reaper? More info here:
https://www.reaper.fm/videos.php#iLC1HjLXLd0
All relevant Input + Monitoring options can also be accessed at once via right-click on the Record Arm button on a Track.
As far as I know the Tascam 16x8 also comes with a dedicated Control Panel Software. So check the routing and settings there, as well.