How do I transfer old iTunes to Apple Music on my iMac?

I know there are a gazillion posts here & elsewhere on the web asking questions about transferring itunes playlists to Music, which is why I'm here. It's like looking for a needle in a haystack, trying to find an answer to my problem. I've followed the basic instructions without success.

So far, all of my playlists show up in Music. All the songs are listed there, however none of the songs can be used "because the original file could not be found."

I've spent years crafting very specific playlists/compilations of individual songs, which make up 90% of my music, so this is not a matter of simply finding an album. It would take days to locate each individual song, many of which are obscure and not necessarily named by artist.

How can I push a magic button and have all the songs located in my playlists?

Note that I used the old itunes library which is located on my hard drive. Also note that I have very limited internet/bandwidth, so any cloud type solutions are out of the question.

Thanks so much to anyone who can help.

[Re-Titled by Moderator]

Original Title: Old itunes to Apple Music issue please help