Can I add a resistor to a 120V-24V power source to convert the amperage from 2.9 to 1.8?
The printer dock determines how much current is drawn, you don't need to add a resistor.

Adding a resistor wouldn't help but if you play around with this, expect smoke, fire and burning shards embedded in both eyes - unless you use high-power resistors, probably lots in parallel.

70 W / 24 V = 2.9 A so the power supply should be able to supply 1.8 A without a problem.

Sometimes (often?) cheap power supplies are made or sold by people who basically lie about their products. I certainly have 9 V wall warts rated for 1 A whose voltage sags to 6 V when drawing 0.5 A.

I would


Measure the voltage with a load on the power supply. 
Try a different power supply.