Czym jest stereotyped sentence
But that, like the preposition but, requires that the matrix clause precede it and be negative:
Nothing would satisfy the child but that Iplace her on my lap. (formal) [l]
More usually an infinitive clause would be used:
Nothing would satisfy the child but for me toplace her on my lap. [la]
In stereotyped sentences, but alone may occur:
It never rains but it pours.