You arent cycled.
Is the water safe to put in a fish? Yes.
Will the water stay safe for your betta? No, because you arent cycled.
You need to cycle the tank. You can either do a "fish in" cycle, where you maintain safe water parameters through regular testing and water changes, or a "fishless" cycle where you dose ammonia to replicate fish waste until your cycle is complete and then add fish.
Each method will typically take 3 to 8 weeks.
Do you have a preference?
I would read up on the nitrogen cycle.