A couple options:
I'm not a pro in these things and my device isn't the Tipo, but the Xperia E. I have a friend who owns a Tipo, and he got a problem similar to this one, so I'm trying to help you out
Allright, first of all, I think
a SIN error means that Flashtool cannot flash the System files, which are the most important thing in a stock ftf(after the Kernel!), but please correct me if I'm wrong!
You have a couple of options you can try:
1. See this link: http://forum.xda-developers.com/show....php?t=2001567
and scroll all the way down to the end of the first post, it says: What to do if you get sin header verification error? That is exactly what you're facing! Follow the instructions.
2. Find a relock-bootloader file for the tipo and flash it with Flashtool( if you can't find anything, or if you don't know what the heck I'm talking about, just ask me for it, and I will try to help you!) If it works try to flash the stock .ftf again. If that doesn't help try to update your system with Sony Update Service, google it, you can find it on the official Sony website!)
3. Repair with Sony PC Companion 2.1 and flash the .ftf file again!
But I think that option 1 will work for you, because I faced the same problem when I had my friends Tipo to root it.
P.S. Use Google! When I searched for: sony tipo st21i ftf, my second result was the link in step 1... But nevermind, I'm happy to help people, so don't bother