“The only reason that you can’t stay on your feet is your hesitation. I never hesitate”
So basically what Kagami told Marinette is that the reason she can’t get Adrien is that she always hesitates at the perfect opportunities. And honestly, I agree.
Marinette has had plenty of chances to tell Adrien how she feels or to get to know him better but she mostly blows it.
The difference between Kagami and Marinette is that Kagami can talk to Adrien and isn’t afraid to make her feelings for him known. Marinette, on the other hand, is the opposite. Even with the encouragement and help of her friends, Marinette still can’t interact with Adrien.
Kagami is giving Marinette some advice while also making her feelings known. Unless Marinette finally gets the guts to confess to Adrien, Kagami is going to get to him first.