The autocorrect works but I only get the red squiggly line, and I have to right click to correct the word. Is there a way to make it immediately correct the word?
The red squiggly line is from Spellcheck.
To add a word to AutoCorrect, click Tools - AutoCorrect - AutoCorrect options. In the dialogue window that opens select the Replace tab and make sure you have your document language selected in the Replacements and Exceptions field at the top. In the Replace field enter the wrong spelling you want to replace; in the With field, enter the correct spelling then click the New button. Continue adding other replacements then click OK to exit.
You can read Help, also the Writer Guide downloadable from English documentation | LibreOffice Documentation - Your documentation for LibreOffice has a more structured approach to understanding LibreOffice Writer.
Can I make Spellcheck automatically recorrect the words then?
No. Spellcheck and AutoCorrect are two different mechanisms.
AutoCorrect will auto-replace whatever you have defined as the replacement: Spellcheck will compare your text with defined phrases in your dictionary, and (optionally) suggest corrections.
Please re-read @EarnestAl’s Reply.
The spell checker only marks words as possibly badly spelled when they aren’t in the dictionary. That doesn’t mean that the spelling is wrong. There is simply no dictionary that contains all possible words of a language, including the jargon of science and professions. It marks words that need your attention, that’s all.
Smartphones nowadays have an autocorrect function that often goes over the top, it will replace correctly spelled words that aren’t in its dictionary by fancy words that sometimes aren’t remotely like what you typed in the first place. Quite annoying really.