What are the reasons for deeds not being accepted, especially with the presence of satanic whisperings related to interpreting a dream of praying while sitting without an excuse, and lacking concentration in prayer, along with excessive thinking during it?

Interpreting dreams is not within our specialized domain, so we do not interpret them, nor is it necessary to definitively interpret them, nor can a legal ruling be based upon them. We advise you to fear Allah, beware of the whisperings of Satan, and diligently perform the obligatory acts of worship with sincerity, especially the prescribed prayers. Reasons for the non-acceptance of deeds include lack of sincerity and contravention of the Sunnah, for Allah only accepts deeds performed purely for His sake and in accordance with the Sunnah of His Prophet. Allah Almighty says: "Say, 'I am only a human being like you, to whom revelation is sent that your God is one God. So whoever hopes for the meeting with his Lord, let him do righteous deeds and not associate anyone in the worship of his Lord.'" And the Messenger of Allah, peace and blessings be upon him, said: "I am the most self-sufficient of partners concerning shirk (polytheism). Whoever performs a deed in which he associates another with Me, I abandon him and his shirk." Ibn Abi Zayd al-Maliki said: "Neither words nor deeds are complete except with intention, and neither words, deeds, nor intention are complete except with conformity to the Sunnah."