First of all, the H1 tag usually stands for the title and name of any piece of information.
H1 tags greatly contribute to the visibility of sites. This is what Google users often see first among the search results. That is the reason why they always contain keywords. H1 tag missing issue means that such a vital element is not present in the body of the site.
If it is missing in general, start with writing a new one. Remember, it should be unique, Keyword rich, creative, and relevant. This type of title shall clearly describe the website, assuming people choose whether to visit a page or not by seeing only this information.
If there is a problem with the code, find the respective field and add this special <h1> </h1> tag to signal a web browser how to display this part of the content.