There are a number of possible answers to this.
Maybe the fire was not taken from the altar, that is, the altar of burnt offering (Leviticus 4:25), in the court of the tabernacle, as is prescribed for the offering of incense in Leviticus 16:12.
Perhaps the incense had not been prepared according to the recipe prescribed in Exodus 30:34–38.
1 Now Nadab and Abihu, the sons of Aaron, each took his censer and put fire in it and laid incense on it and offered unauthorized fire before the LORD, which he had not commanded them.