We had to sign the petition. (Make it negative)

Must/ have to থাকলে negative করতে হলে Structure হচ্ছে -
i)    cannot but + verb এর base form
ii)    cannot help + verb+ing

একই ভাবে had to থাকলে –
iii)    could not but + verb এর base form
iv)    could not help + verb+ing

তাই সঠিক negative form হবে - We could not help signing the petition.