Is buying a house by installments permissible or forbidden, and what is the interpretation of the supplication, "O Allah, loosen a knot from my tongue so that they may understand my speech"?

If you are asking about purchasing a house by installments, then installment selling is originally permissible if it adheres to the necessary Shariah regulations.

As for the mentioned supplication: "My Lord, expand for me my breast [with assurance] and ease for me my task and untie the knot from my tongue that they may understand my speech," it is quoted from the words of Allah Almighty, recounting Musa (peace be upon him): (And untie the knot from my tongue That they may understand my speech) [Taha: 27-28]. The meaning of the verse is: "Unleash my tongue in speech and remove the impediment and defect within it so that they may understand my words and discourse in conveying the message." He had a slight stutter or lisp in his tongue.

Among the subtleties of Musa's supplication is that he did not ask for the complete removal of the impediment, but rather he asked for the removal of the defect so that they could understand what he intended from them, which was the extent of the need.