What's the red exclamation mark on Apple's SMS?

The information icon on Apple's mobile phone has an exclamation point, because the imessage information is not opened. The solution is as follows:

Tool: iPhone6s

Operating system: iOS 13.4. 1.

1. In the interface on the desktop, the information icon has an exclamation point, as shown.

2. Click the Settings icon on the desktop.

3. Open the phone settings interface and click the information option.

4. Open the information interface and click the switch button in the imessage information bar.

5. At this point, the imessage information bar is light green after it is successfully opened.

6. However, when you exit the desktop, you can see that the exclamation point on the SMS of Apple's mobile phone has been deleted.