Backlog Cannot tap Mox Amber while you control no legendary creatures or planeswalkers

[Mikail]

According to the Mox Amber official rulings:

If you control no legendary creatures or planeswalkers, you can activate Mox Amber’s ability, but you won’t add any mana. (2018-04-27)

However, it is not possible to tap it to its own ability on Magic Online if you control no legendary creatures or planeswalkers, it is just inactive.