微信小程序 wx.onShareAppMessage is not a function

The error message "wx.onShareAppMessage is not a function" suggests that the method "onShareAppMessage" is not available in the current context.

This issue may occur if you are trying to call the `onShareAppMessage` method in a wrong context or if it's not supported in the version of WeChat Mini Program you are using.

To resolve this issue, make sure you are calling the `onShareAppMessage` method within the correct context. The `onShareAppMessage` method is typically used inside a Page object to define custom sharing behavior.

If you are using an older version of WeChat Mini Program, it's possible that the `onShareAppMessage` method is not supported. In that case, you can consider using the `button` component with the `open-type="share"` attribute to enable sharing functionality.

If the issue persists, please provide more details about your code implementation, the WeChat Mini Program version you are using, and any relevant error messages for further assistance.