Your GST Identification Number is on your GST registration certificate (Form GST REG-06) — 15 characters, no spaces.
Reading the format
2-digit state code, your 10-character PAN, a 1-digit entity code, the letter Z, and a checksum character.
Why we check it first
Each GSTIN can only be registered to one ClayBill company — checking it now saves you re-entering everything if it's already taken.
PAN is optional here
You can add it now or later from Company Settings — you'll need it before generating real e-invoices or e-way bills.