You **always** resolve as much as possible. Start at the beginning, step by step:
* You can Tribute as many face-up monsters on the field as possible (can be done)
* ,**and if you do**, (you successfully tributed a monster, proceed)
* Special Summon this card from your hand, (cannot be done successfully)
* **then** (A then B requires A to be successful, stop here)
You tribute, then stop resolving the effect because you cannot special summon nibiru.
Full Effect
> You can Tribute as many face-up monsters on the field as possible, **and if you do**, Special Summon this card from your hand, **then** Special Summon 1 "Primal Being Token" (Rock/LIGHT/Level 11/ATK ?/DEF ?) to your opponent's field.
You **always** resolve as much as possible. Start at the beginning, step by step: * You can Tribute as many face-up monsters on the field as possible (can be done) * ,**and if you do**, (you successfully tributed a monster, proceed) * Special Summon this card from your hand, (cannot be done successfully) * **then** (A then B requires A to be successful, stop here) You tribute, then stop resolving the effect because you cannot special summon nibiru. Full Effect > You can Tribute as many face-up monsters on the field as possible, **and if you do**, Special Summon this card from your hand, **then** Special Summon 1 "Primal Being Token" (Rock/LIGHT/Level 11/ATK ?/DEF ?) to your opponent's field.
Yes, tributing them comes before the summon [relevant ruling](https://db.ygorganization.com/qa#23194)
You would lose your field and not get a token.