Why can't Apple send text messages with a secondary card?

Because the secondary card is not set to send short messages, the solution is as follows:

1. First open the settings on the home page, as shown below.

2. Then find "Cellular Network" in the settings and click on it.

3. Then, in the cellular network, open the settings of two cards and click on two cards respectively.

4. Click the data flow card at this time, then open this card and click "Enable this number".

5. After the two cards are enabled, enter the SMS page. Find the place pointed by the arrow and click on the green box.

6. Click the green box to find the card that can be replaced. Choosing a secondary card here means sending text messages with a secondary card.