H1 too long issue means that h1 tag length on some pages exceeds 70 characters in length.
Why it is important
Of all the headings in an HTML document, the h1 tag is the one with the highest priority, while h6 has the lowest possible priority level. Lower level headings and the text that they refer to are part of higher-level headings, hence, the totality of headings in a document forms a hierarchical structure similar to the outline of a document. While h2-h6 headings ale the elements of this outline, the h1 plays the role of the headline of the page or article. Headings are formatted using visual styles that convey their relative importance – h1 typically has the greatest font size.
An overly long h1 tag may lower the power of the text embedded in this tag.
Search Engine Optimization (SEO) Starter Guide made by Google recommends avoiding: excessive use of heading tags on a page and very long headings.
How to check the issue
Head to https://codebeautify.org/source-code-viewer in order to open the source code of the page. Alternatively, use the functionality of your browser. In Chrome, it is enough to right-click the page and pick “View page source” from the context menu.
In the source code, locate text sequences flanked by <h1> which designate h1 tags.
How to fix the issue
Edit h1 tags so that they have 20-70 characters in length, as recommended.
Check your website for long h1 tags
Audit your website to detect pages that have too long h1 tags