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Oath of Nissa will not let you use mana from nykthos' ability to be spent as any color, same goes for mana from any source that doesn't just tap to produce a mana.

For starters, report bugs in the monthly bug report thread. Report them there.
Second, Oath's ability only allows you to spend mana of any color to cast planeswalkers.
Third, it works perfectly fine for me if I tap Nykthos while casting the spell - it didn't seem to want to work when I tapped Nykthos for mana beforehand. Just tap your lands when casting the spell rather than beforehand and you'll be fine - it's how you should play anyhow.

This should go in the bug reports thread as mentioned. But to clarify the post above - this is a known issue with Oath of Nissa - it does not work correctly with floating mana.

reverendvile wrote:Third, it works perfectly fine for me if I tap Nykthos while casting the spell - it didn't seem to want to work when I tapped Nykthos for mana beforehand. Just tap your lands when casting the spell rather than beforehand and you'll be fine - it's how you should play anyhow.

Except for many possible situations where you might want or need to use floating mana and where Oath of Nissa isn't working properly. If the card isn't allowing you to use it to its full extent in accordance with the rules of the game then that's a bug. How one "should play" and how one "could play" are two different things.