ddC and dC3 indicate the AddWash hatch latch on Samsung models that include it. The main door may be closed and still not provide the Add Door confirmation.
The Samsung documentation distinguishes that additional laundry inlet. If your model does not have AddWash, check the code and the manual before following this path.
Check the correct hatch
Pause using the intended controls and wait for the machine to allow you to handle Add Door. Hot water, the water level, and spinning can prevent it from opening even if it is a small hatch.
When it is safe, check whether a visible garment or object is preventing it from closing. Close it as the manual instructs, without force or pressure on the latch. If there is damage to the hatch or its latch, stop trying.
Do not copy unlock combinations from another panel or bypass sensors. If ddC/dC3 appears again with the area clear, note whether it happened at startup or after adding laundry, and request inspection of the corresponding latch.
Tell support that the warning comes from AddWash; saying only “the door is closed” can hide which latch is failing.