Why can't Apple 7 send invisible messages? What is the reason?

The 3D touch function is not turned on.

The steps to turn on the 3D touch function are as follows:

Open settings-> general;

Turn on the auxiliary function;

Enable 3D touch, and select Open when 3D touch is successfully opened.

The step of sending invisible information are as follow:

Open the message, select the recipient, enter the content in the text box, and press and hold the send symbol on the right;

Finger up selection is sent with invisible effect;

Invisible message sent successfully.