Make sure to set up a new local or existing Microsoft account, on the computer sharing the files. You can't use the same Microsoft account that is logged in on that computer, took me some time to figure that one out. Doesn't really matter which account you login with anyway, luckily. Anyway, make sure to give this user the proper permissions, then when connecting to this computer use the login credentials for that new user.
I literally just spent the day setting this up my self, I'll make some pictures for you. (making them now if needed)
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Edit: I've now added some quick images with step by step, of how to setup the permissions. I hope this helps, hope you guy's don't cringe too hard on my bad choice of names for my rigs. Once you've set those up, make sure to login with the correct credentials, and with an account that is not logged in on the computer sharing the files. If the "main" account is not logged in, which is probably the same account you're using on the other computer, you still can't use this to login either. Gives that nasty message saying you have no file access for whatever reason.
Bonus: Do you see the "Manage" button on the file explorer? Click it and go into "Shared Folders" and have a look at all the useful information available, such as who are connected, which folders/drives are open, what is being shared, how long clients have been connected and you can also disconnect clients here. Quite useful in the long run
Have fun