There are generally a few things that can result in publishing errors.
Your repository must be named based on the username of your GitHub account, otherwise the website will not build properly. Note, however, that this can easily be remedied:
[yourusername].github.ioOnce you’ve finished, go to the “Actions” tab to check the build progress.
Every .md file in your site has two sets of 3 hyphens (---), as the first line of the .md file and as a divider between the “front matter” metadata and the main part of the .md file. These need to be positioned and typed exactly.
Check each recent file and look at both instances of the 3-hyphen lines
There are two characters that can create problems for the metadata between the two sets of hyphens: quotes and colons.
Each front-matter line is the name of a type of information, a colon, and the actual information that gets used. For instance, the title of a page is set in this front matter:
title: Title of Page
If the page title actually has a colon in it, that can break the publish. Add quotes around the whole title to make the second colon show up as a colon instead of being something the computer reads as a computational marker.
title: Title of Page: this is a page that does something
title: "Title of Page: this is a page that does something"
Too many quotes can also be an issue. If the page title has quotes around it, and there are also quotes in the title, that can create a problem. Use apostrophes as quotes inside the double quotes
title: "Title of Page: "Some word" is in quotes"
title: "Title of Page: 'Some word' is in quotes"