Now that character SHERLOCK HOLMES is no longer under copyright and is in public domain, can I bring him into my fantasy story where the Protagonist meets him to get some help?
This question was asked previously: Is Sherlock Holmes public domain for use in a story? but a US court case in 2013 (and it's failed appeal in 2014) has since established that Sherlock Holmes, as depicted in all but the last-published stories, has lapsed from US copyright and is now in the public domain.
What this means is that the early version of the character is public domain, however character details that were introduced in later stories are still under copyright.
The remaining titles are due to lapse between 2019 and 2023.
The copyright has already lapsed in the UK.
I am not a lawyer. The general points I'll make have all sorts of legal specifics you need to look up. With that being said:
Yes, you can reference or even use the character. When TV and film producers do the same, they don't pay a penny. In theory, you should be careful which stories you reference. They have different publication dates, and the dates on which they enter the public domain also vary by region. And there is one other pitfall: copyrightable original elements of derivative works are protected, so don't copy those. In other words, if you steal something you must know where it originated.