I checked eCAS few days ago and saw that CIC sent me a correspondence. I thought it was for FP request but I was surprised when the mail came and they are asking for my SIN. Is this normal? Please enlighten me. Thank you.
No, this is NOT normal. It probably means you failed to enter your SIN in the application, or it was illegible, or you made a mistake in it so the number entered is not valid.
As
@Bargeld suggested: wait and respond to what IRCC actually sends you. Should get that fairly soon.
I honestly do not remember if I've provided it or not.
I am guessing you did not make and keep a complete copy of your application. Otherwise you would look at your copy and see what you submitted in response to item 12. a)
For future reference and for any others who could use the reminder:
BE SURE TO MAKE AND KEEP A PHOTOCOPY OF EVERYTHING YOU SEND TO IRCC. ALWAYS. Like tax returns. Everything. Completely. After it has been signed and dated. Really.