If the Image Alt text is are missing from images within your post/page content – Rank Math can help you automate the process of adding them. Check the settings for that in WordPress Dashboard > Rank Math > General Settings > Images.
If the alt text is missing from the images added using your theme, you will need to edit the PHP files for that. Please get in touch with your theme provider and they will be able to provide you with exact instructions on that.
Hope that helps and please do not hesitate to let us know if you need our assistance with anything else.