The {tab} tags are not being converted

The Tabs & Accordions plugin is not published

Check if the Tabs & Accordions plugin is published.

Go to Extensions >> Plugin manager and search for the system plugin System - Regular Labs - Tabs & Accordions. Then publish it.

There is markup on the {tab} tags

It is fine to give styles to the titles within the {tab} tags, like: {tab title="My Title"}
But giving styles to other parts of the tag, or the entire tag, can cause problems, like: {tab title="My Title"}

If you have copy/pasted code directly into your editor, you might also have copied along hidden HTML tags. So this will also cause the {tab} tags to break:

{<span>tab</span> ...}

Remove the extra styling or other html tags around and in the {tab} tags that might cause problems.