Letter of weakness

Look here: https://opentuition.com/acca/f8/f8-typical-internal-control-systems/ It sets out internal control deficiencies, possible consequences and how to improve the internal control system.

is it Limitations of iCS?

Letter of weakness contains specific problems with the client's system. You might be talking about inherent limitations of internal control as a general concept. That's is also covered in the notes.

oh, is it also known as 'Letter of Recommendation' ?

No.

is there something called Letter of Recommendation at all or my teacher just passed a random stupid heading for letter of weakness? Because it has Weakness, Consequences and Recommendations

I've never heard of a 'letter of recommendation'. It might be that your teacher used that term for a letter of weakness. Some firms of accountants might even use that term I suppose, but I don't think its common usage.