What tenkeyless means
Tenkeyless, usually written TKL, is a full size keyboard with the number pad removed. The name comes from those ten digit keys, though the pad actually holds seventeen. What remains is 87 keys on ANSI: the alphanumeric block, the full function row, and the navigation cluster with the arrows.
TKL is the most common layout among mechanical keyboards because it keeps every key people use daily while giving back around 15cm of desk space, which matters more to anyone who moves a mouse a lot.
Test the cluster, not just the letters
On a TKL the navigation cluster is the part that gets least use and therefore hides faults longest. Insert, Scroll Lock and Pause can sit dead for months without anyone noticing. Work across them deliberately here rather than typing a sentence and calling the board healthy.
The arrow keys are the opposite case: heavily used, and on a gaming board often the first switches to wear, because they take the same abuse as WASD without the same quality of switch in cheaper models.
If you need the number pad back
Most operating systems accept a separate USB number pad, which shows up as its own keyboard and sends the same codes. If you are testing one of those, use the full size layout on this page instead so the pad is drawn.
A TKL with a dead arrow key is worth testing under the full size layout too, to confirm the board is not sending number pad codes for the arrows, which happens when Num Lock is misreported by a firmware layer.