First, please could you confirm if you're trying to share music to your Instagram stories but are redirected to sign in to Instagram instead? This thread concerns this specific issue.
If so, could you make sure that you have reinstalled both Spotify and Instagram, and that when you are trying to share to your Instagram Stories you are logged into both your Spotify and Instagram accounts?
If that doesn't work for you, please could you leave the following information:
Device and OS
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Re: I can’t get song share via Instagram stories to work
I have exactly the same problem on iPhone XS! Everything is up do date, I’m not sure why Instagram asks me to sign in when I try to share from Spotify. Once I do sign in, it just brings me to the main Instagram page and the song isn’t shared.