SigningCredentials on .NET Core is different from .NET 4.0
The equivalent code in .NET Core would be:
var key = Encoding.UTF8.GetBytes(accessKey);
var signingKey = new SymmetricSecurityKey(key);
var signingCredentials = new SigningCredentials(signingKey, SecurityAlgorithms.HmacSha256);
The class names have changed a little, but everything should work the same. Make sure you are using the latest package versions.