What is the reason why imessage on Apple's mobile phone can't be activated?

Cannot activate because of a setup problem. There are several steps about setting up:

Step 1: As shown in the figure below, when iMessage is successfully activated, you can see that it has been opened in the Settings-Information list.

Step 2: Sometimes activating iMessage will encounter problems that cannot be activated. In general, you can try to activate it several times first.

Step 3: If it still doesn't work, please turn off the iPhone, then turn it on, and then activate it.

Step 4: If the problem cannot be solved after restarting, you can try to turn off the wireless network and activate the iMessage information service directly with the mobile phone network. You can directly open the control center, click turn off wireless, and then try to activate.

Step 5: If activation through wireless network or mobile network is unsuccessful, you can also try to restore the network settings of iPhone at this time. In Settings-General-Restore, you can choose to restore the network settings, as shown in the figure.

Step 6: After the recovery is successful, reconnect the network to see if the iMessage service can be successfully activated. If it still doesn't work, the suggestion given at this time is to activate the iMessage service another time.