DIAGNOSIS This is almost always a battery connection issue, not a dead tool. The contacts on the driver or the battery can be too tight or have packing debris.
FIX First, take the battery off. Look into the battery port on the driver. You'll see two flat, spring-loaded metal contacts at the top and two wider contact pads at the bottom. Press on the springy top contacts with your finger to make sure they're moving freely and not stuck. Then, look at the terminals on the battery itself. Give both the battery and the tool's contacts a quick wipe with a clean, dry cloth to remove any plastic film residue or dust. Re-seat the battery with a firm, solid push until it clicks.
IF THAT DIDN'T WORK Try a different 20V MAX DeWalt battery if you have one. If it works, the problem is your original battery. If it still doesn't work with a known-good battery, the tool's internal trigger or board might have a factory defect.
HEADS UP Don't try to bend or modify the contacts. If they're physically damaged or the tool is DOA, it's a warranty claim. The DCF787 has a 3-year limited warranty from the date of purchase.