The most likely reason why you can't access an ebook or journal article is because the University doesn't have access to the ebook or journal, or doesn't have access on the website you have found it on. 

For example I have found the ebook  The Economics of Climate Change on the De Gruyter ebook platform but I don't have access even through I am signed in:

• At the top left of the screen it shows that I am "Authenticated with the University of St Andrews", this means I am signed it to the De Gruyter website.
• Above the title it states "Unlicensed", this means that the University of St Andrews doesn't have access to this ebook on the De Gruyter site.

This doesn't mean that the University doesn't have access to the ebook, just that it isn't available to University of St Andrews users on this site.  There are two options for checking if you have access to the ebook or journal article on another site:

1. Use Library search and search using the title of the ebook or journal article.
2. Install the Lean Library browser extension which will detect if we have access on another site.  See the question and answer on Lean Library for more information.

Using Library search I have found that we have access to this title, it's available to me on the Oxford Academic website:

The green open padlocks and the text "view chapter" show that I have access. 

If you use Library search or Lean Library to check for ebooks or journal articles, and the Library has access to the ebook or journal article, using the links provided to you will ensure you are logged into these resources through the University's single sign-on.

If you can't access an ebook or journal article, and you think you should have access, please contact the Library by emailing Library@st-andrews.ac.uk , or speak to the staff at the Helpdesk in the Main Library, or use the online chat service.