My steps, if anyone has to replicate. I'm not sure if everyone will start from here... it's possible that you already have the prerequisite installed from another game. But I'll give my fix path in case anyone starts from the same place I did.
For me, what I saw to start: When running, a brief spin, and then about ten seconds later, a change back to the green play button. Complete no-go.
My fix path went:
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Started in Desktop mode, for maximum troubleshooting flexibility.
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Going to Manage > Properties, I changed BG III compatibility option to Proton Experimental.
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Tried relaunching game... this time, an error returned that I needed to run the game along with Windows Net Desktop runtime (.net 6.0.20).
This error had its own link button, but in case you don't get the link or error, but still want to see if this is your fix, the link took me here for the download.
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I added that downloaded file as a non-steam game in Steam, set its compatibility to Proton Experimental, then I ran it and installed it. (Note: For stuff like this, leave it in your Steam library after you're done with these steps... if you delete it later, it takes out the file path and it will stop working, and you'll have to do all this all over again.)
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Every person is going to get a unique directory number created for them in their file structure for this file. Find it by following the path in your file manager (default file manager is Dolphin): /home/deck/.local/share/Steam/steamapps/compatdata/
Then sort by "last modified," and the file folder you're looking for will pop to the top (it should say something like "last modified two minutes ago," "last modified just now," or something like that... because you just did this, so you know this is the correct one). Write this number down, it's the unique-to-you location id I was talking about earlier.
Finally, go back to Baldur's Gate III and the Manage > Properties. In the launch options, paste exactly this command, except substitute your unique ID number in place of the string of Xs here:
STEAM_COMPAT_DATA_PATH=/home/deck/.local/share/Steam/steamapps/compatdata/XXXXXXX %command%
(Note: there is a space between your ID number and %command%.)
And if your issue was for missing the Net prerequisite, then this should get it running for you. Have fun!
I'm kind of hoping that Larian realizes they forgot this, and just patches a fix to auto-install and run this with the game naturally, so that there isn't a need to go through this rigmarole.
They've been pushing for this launch date for weeks now, and to me, them leaving it out is the developer equivalent of a major league baseball player running out to the field, but leaving his glove in the dugout. Like, how did you forget that? lol