Hello!
Thank you for making a bug report.
I’m not sure the circumstances of your testing. I see in the image you have attached that Blue-eyes Spirit Dragon is currently on your opponent’s field. Spirit Dragon prevents more than 1 monster from being summoned at a time. Nebula Dragon summons both monsters at the same time in order to resolve its effect, and it is being prevented from activating since Blue-eyes Spirit Dragon is on the field.
If you enter a game where your opponent does not have Blue-eyes Spirit dragon on the field, you should be able to summon it. I have tested Nebula Dragon using the OCG and TCG rule sets.
Let me know if you continue to see this problem and we will look into it again.
Thank you!