Myanmar is a beautiful country. It is rich in natural resources.
If I rewrite this sentence as
Myanmar is not only a beautiful country but also rich in natural resources.
Will it be grammatically correct? And does the given sentence make sense?
That's fine and makes sense. Putting in a comma after the first attribute ("beautiful country") is discretionary and not usually necessary (or desirable) unless the sentence is very long or you really want to emphasize the separateness of the two things, for instance when the spoken sentence might include a dramatic pause after the first item. ("This is not only a top quality shirt, but also very affordable!")
Yes it is correct.
It can also be written as:
Not only is Myanmar a beautiful country but also rich in natural resources.