You can only add words to the default custom dictionary. Ensure that dictionary is configured for all languages - or at least for the language you are typing in.

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Graham Mayor - MS MVP (Word) (2002-2019)
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Thanks gmayor for the help!
Yeah, I know that. I've added in that dictionary, the word is there but it doesn't appear in the spelling checker.
I've already configured the dictionary for all languages and mine too.
But the issue continues.
Thanks anyway!!