This case applies only if you have already registered to Instantgo in the past and already have an account. If this is the case and when you are trying to sign in back to the app you get a message that your username is taken, it means that you are not using the same email address that you used when you first registered and therefore the platform thinks that you want to register as a new user. This is why the system asks for a username. If you were using the email address that you initially used, you would not have the option of choosing a username as you already have one.
If this is happening to you, try to remember which email address you could potentially have used in the past and try it. If you still can't make it work, contact support which will try to find the email you have used in the past by using your username.