This is because the mac version is too old to support Battle.net. Updating the mac system can solve the problem.

the method of updating the Mac system is as follows:

1. open the App Store on the MAC computer, and you can see the update of the Mac OS 11.11 Yosemite system in the boutique column, just click the "download" button, as shown in the figure:

2. After clicking "download" in the App Store, you will be prompted whether to download and update the Mac OS 11.11 system, and click "Continue".

3. Next, you need to enter the Apple ID and password, as shown in the figure:

Then the system can automatically download the latest 11.11 system. You can find it in "Purchased Items" in the App Store and see its download progress.

4. System installation

Whether the APP store is updated or an installer is installed, open it and install it as shown in the following figure.

Next, select the disk to be installed. After that, you need to enter the login password of the current user.

5. After waiting for processing, you will be prompted to restart the system, and click the "Restart" button to continue.

6. When the system is to be updated, ensure that the power of the computer is more than 51%, or directly link the power cord to upgrade. It takes about 1 hours to upgrade the system once.