Can I use a 20V or 24V to power up a piano that needs 21V?
The difference of 1.5A or 3A does not matter. I.e. the adapter should be at least the Amperage requested (>1.5A).

However, for the voltage, it should in principle not be exceeded. As Kokachi wrote, first try 20V. 

If you dare to try 24V (not advised), you run into the risk of burning the piano's electronics. But even if not (immediately), it results in more heat, which reduces the lifetime (probably).

Also check the polarity, if needed you can easily change this by resoldering the adapter cables.