I have the same issue here, I followed a online video to create Identity propject. It uses below .net core 8, but my project is 8. when I run it, it shows the same error. My _jwtSettings.Key was 19 chars. I change it to longer. Than it works.

var symmetricSecurityKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(_jwtSettings.Key));