Hello there,
I had the same problem:
The library exists but I got errors messages after execution of my Flow (with action "Send http request"). I changed severely the name of my library, I checked If I'm effectively Owner and I put myself as Owner (again) with Full Control to try if it will change something... Still same problem 😟.
But I got an alternative as solution 😎, Use the "ID" and not the "ListName" of the library:
1) So, to get the ID of your library (there are many ways), go to the Settings of the Library and check the URL.
On the end of the URL you've got letters and digits - it begins with %7B and ends with %7D.
-->the letters and digits between these two is the ID of your Library.

2) Now go to your Flow back, to action "Send http request..."
and now we are trying to get the library no more By Title but By ID so we will change this:
Replace "GetByTitle" with "GetByID" and replace the name of your library the ID like this ('xxxxxxx')

I hope this solution will help you and all the other people who are struggling with this error 😁