There isn't any book that has a Myconid as a playable race. You can use the homebrew creator without buying any particular book. The main reason to buy anything is if you know of a race that's got similar features that you'd like to copy, since you can use that race as a template. Since I don't know what you're trying to do, I can't suggest any - but it's not necessary anyway.
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If you're not willing or able to to discuss in good faith, then don't be surprised if I don't respond, there are better things in life for me to do than humour you. This signature is that response.
I have an idea to build a Myconid character. What book, if any, do I need to buy in order to build this using DnDBeyond?
There isn't any book that has a Myconid as a playable race. You can use the homebrew creator without buying any particular book. The main reason to buy anything is if you know of a race that's got similar features that you'd like to copy, since you can use that race as a template. Since I don't know what you're trying to do, I can't suggest any - but it's not necessary anyway.
If you're not willing or able to to discuss in good faith, then don't be surprised if I don't respond, there are better things in life for me to do than humour you. This signature is that response.
There's no playable Myconid race in any of the books.