错误在这里...

byte[] signingKey = GetSignatureKey(Key, "20180308", "us-east-1", "s3");

此行应为...

byte[] signingKey = GetSignatureKey(Secret, "20180308", "us-east-1", "s3");

您使用的是aws-access-key-id而不是密钥来生成签名密钥.

You're using the aws-access-key-id rather than the secret key to generate your signing key.