The prompt will occasionally pop up even if you can't take the action. (Wizard casts Magic Missile on target who responds with Shield. Prompt comes up for Counterspell on Shield. I click 'Yes', but the Counterspell doesn't happen as Solasta won't let you cast two leveled spells a turn.) This is more bug territory than anything else, but any reaction system relies on a robust system of conditional checks. Make sure it works, please.
100% agree that the prompts should only trigger when a reaction is actually viable.
It looks like Solasta must have hardcoded not allowing 2x level spells per turn, which is actually an error in rules implementation as there's no rule against that.
In Tabletop you can cast 2x levelled spells per turn. The casting limitation rule applies when you've used a Bonus Action to cast - i.e. once you've bonus action casted a levelled spell, you can't cast anything BUT an action cantrip that turn. But if you have a legit way to cast 2 levelled spells - i.e. via reaction or action surge, you totally can do it.
I would say, I'd be 150% okay for Larian to adjust some reactions so they pop-up less. I.e. a Diviner's Portent per the rules would be insanely disruptive since it's applicable on every single roll, friend or foe.