Lack in Obligatory Prayers Is Compensated by Optional Prayers
4-10-2018 | IslamWeb
Question:
I heard that our Nafl prayers wont be accepted if we left any fard prayers in our life,is it true? Does the nafl prayers include Salatut Tawbaah and salatut tasbeeh that won't be accepted too even if we asked for forgiveness of those fard prayers?
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
There is no doubt that abandoning an obligatory prayer is a serious matter, but we do not know any evidence from the Shari'ah that suggests that a Nafl prayer cannot be accepted if someone has abandoned an obligatory prayer.
Rather, there is evidence in the Shari'ah that indicates that if someone has any deficiency in the obligatory prayers, on the Day of Resurrection, he will compensate for it with the voluntary (optional) prayers.
Abu Hurayrah 
narrated that the Prophet said: “The first thing about which the people will be called to account out of their actions on the Day of Judgment is the prayer. Our Lord, the Exalted, will say to the angels - though He knows better: Look into the prayer of My servant and see whether he has offered it perfectly or imperfectly. If it is perfect, that will be recorded perfect. If it is defective, He will say: See if there are some optional prayers offered by My servant. If there are optional prayers to his credit, He will say: Compensate the obligatory prayer by the optional prayer for My servant. Then all the actions will be considered similarly.” [Abu Dawood]
The optional prayer that is mentioned in the Hadeeth includes the Sunan prayers as well as the general optional prayers.
The author of Tuhfatu al-Ahwadhi wrote: "See if there are some optional prayers offered by My servant.”; in his book of records, meaning any Sunnah or Nafl prayer as it is apparent from the context…..”[End of quote]
Similarly, the deficiency in the obligatory prayer in this Hadeeth is likely to mean abandoning it completely.
Awn al-Ma’bud reads:
“Al-Iraqi said in Sharh at-Tirmithi: What is mentioned in compensating the obligatory prayer of the person with the optional prayer is probable to mean the deficiency in the Sunan and the legitimate positions [or acts] that are recommended; like Khushu’ (attentiveness and submissiveness in prayer), Athkaar (legislated mention), and supplications and that he gets the reward of that for the obligatory prayer even if he did not do it in the obligatory prayer, and he only did it in the optional prayer. It is also probable that what is meant is the obligatory prayers that are totally abandoned and which a person did not pray at all, so he is compensated for it by optional prayers as Allah accepts the correct prayers instead of the obligatory ones; Allah is free to do whatever He Wishes, to Him belongs all Favor. Rather, Allah may forgive a person even though the latter did not do anything, neither an obligatory nor an optional prayer. ‘Then all the actions will be considered similarly’; meaning if there is a deficiency in the obligatory deed, it will be compensated for by the optional deed. A narration by Ibn Maajah reads: “….then all other obligatory acts will be dealt with similarly…..” [End of quote]
The prayer of repentance is one of the optional prayers; we mean by it the two Rak'ahs prayer mentioned in the Hadeeth: "There is no servant (of Allah) who commits a sin, and then performs ablution well, and stands and prays two Rak'ahs, and seeks forgiveness of Allah, except that Allah forgives him. He then recited this verse: {" And those who, when they commit an immorality or wrong themselves [by transgression], remember Allah and seek forgiveness for their sins…} [Quran 3:135]” [Abu Daawud and others]
The same thing applies to the Tasabih prayer; the view that we adopt here in Islamweb is that its Hadeeth is not graded less than a Hassan Hadeeth, as it was narrated with various narrations that strengthens it.
According to the view that the Tasabih prayer is permissible, it is among the general optional prayers.
What is important for you to know is that for you your repentance to be valid you must make up the obligatory prayer that you had intentionally abandoned, according to the view of the majority of the scholars.
Allah knows best.