Laura: 'I was said' is incorrect. You should write 'I was told'. 

'Its countryside' is indeed correct. 

'I was once told that its countryside is astonishing' is how I'd write your sentence. 

Hi Joseph! 

Thanks for the correction!

In my sentence I wanted to say that a friend of mine told me once that Scotland was great that is why I said : "I was said once ..." isn't this structure correct to mean a person told me something?

About "its" I was meaning the countryside of Scotland that is why I wrote "its countryside". Isn't it right?

This was the original sentence "I was said once that its countryside was astonishing" 

I'm going to quote a comic strip by Bill Griffith, creator of "Zippy, the Pinhead."

http://tinyurl.com/htsocef

(I'm going to correct the English slightly).

Griffy: Zippy, if you could live anywhere in the world, where would you live?

Zippy: Albany.

Griffy: Albany?

Zippy: Albany.

Griffy: Why Albany?

Zippy: Because I'm in Albany.

(Albany, New York, is a pleasant but unremarkable city of 1.2 million; it's the state capital of New York State).