Hello Simona. Looking at the dialog I would say I that I can see two mistakes:
1. I could book up Wednesday. I this case it means that you would make a reservation which doesn't really make since in this context. we are aiming to say that you are free on Wednesday or in other words, I could do Wednesday
2. Does 3 o'clock convenient for you. Convenient is adjective. When we use do or does we have to use another verb in the sentence which is your sentence lacking. If we wanted to use convenient in the sentence, the sentence would be Is 3 o'clock convenient for you? Since we have given structure of the sentence wold work is the most suitable to use. Does 3 o'clock work for you, which expresses if the given time seems to be suitable for you.

Ahoj Simona, prvé cvičenie máš správne. Ten dialóg sa mi ale moc nepozdáva. Chcela by si to prejst a vymyslieť nový s tými istými slovíčkami alebo máš dane nejaké zadanie?