95 SLS anti-theft problem

I just installed a new engine a few weeks ago and I am getting the message "anti-theftproblem" car might not re-start. It is kinda freaking me out because I do not want to be stuck somewhere if my car wont start. Is this anything serious and how do I fix it? thank you.

U Mite Want To Try Replacing The Alternator. Then Check The Volts. U Should Be Ok After That