Same happened to me,
I am a freelancer too, I purchased a new car few months ago and I got notice same notice from FBR to file Income Tax Returns
I simply hired a guy who did it for me and I had to pay PKR 0 tax
Do not ignore that email, if you do, you may have to pay penalty or there is a chance they will block your bank account aur phir FBR k office dhakkay khany pare gy
You don't have to pay tax, so just file your returns, simple is that, and you will get several benefits if you are filer, you will pay less taxes on purchase of car or land etc, also, you will not be charged for any WHT on bank transactions
there maybe other benefits but I am not aware of them