Try running HDT as admin.
If running as admin doesn't work immediately, rebooting seems to help sometimes.
For more information, see https://github.com/HearthSim/Hearthstone-Deck-Tracker/issues/3143
Why can't I see the deck overlay when playing Hearthstone?
Here are some settings to check to make sure Hearthstone Deck Tracker is working properly.
Written by Andrew Brown
Updated over 6 years ago