These verses relate to the expedition of Tabuk, confronting the Byzantine army.
For more information on the battle of Tabuk, please click on the following article: “Answering Jihad: ‘Fight Against Those Who Do Not Believe’ – Quran 9:29”
9:91 There is not upon the weak or upon the ill or upon those who do not find anything to spend any discomfort when they are sincere to Allah and His Messenger. There is not upon the doers of good any cause [for blame]. And Allah is Forgiving and Merciful.
9:92 Nor [is there blame] upon those who, when they came to you that you might give them mounts, you said, “I can find nothing for you to ride upon.” They turned back while their eyes overflowed with tears out of grief that they could not find something to spend [for the cause of Allah].
The above verses were revealed concerning the Battle of Tabuk, when sincere Muslims at the time were deprived from defending the Muslim community against the aggression of the Byzantine Empire’s army, who were assembling to attack the Muslim community, 1400 years ago. The people that were deprived from joining this were those who were blind, sick or handicapped.
Then there is the second group of people who were ready to defend the community but were unable to participate due to them not having an animal to ride on. This journey to Tabuk was a long trip and the weather was very hot. It was impossible for these companions to walk all the way. One most probably would have died as a result of the sheer heat.
Maulana Muhammad Ali:
“92a. Apparently, what they wanted to join the expedition, and what the Prophet could not find for them, were beasts to ride upon and to carry their provisions and necessaries.” 
Malik Ghulam Farid:
“1208. The verse is general in its application but the persons particularly referred to were seven poor Muslims who were extremely desirous of going to Jihad but did not possess the means and wherewithal to fulfil the wish of their hearts.” 
Scholar Abdullah Yusuf Ali:
“1343 Though active service in person or by contributing resources is expected in emergencies of every person who believes in the Cause, there are some who must necessarily be exempted without the least blame attached to them. Such would be those who are weak in body on account of age, sex, infirmity, or illness. Personal service in their case is out of the question, but they could contribute towards expenses if they are able. But if they are too poor to afford even such assistance, they are excused.
But in all cases the motive must be sincere, and there should be a desire to serve and do such duty as they can. With such motives people are doing good or right in whatever form they express their service: sometimes, in Milton’s words, “they also serve who only stand and wait.”
In any case their purity of motive would get Allah’s grace and forgiveness, and we must not criticise even if we thought they might have done more.
1344 Hamala, yahmilu, here seems to mean: to provide means of transport, viz. mounts (horses, camels, etc.) for riding, and perhaps beasts of burden for carrying equipment and baggage, suitable to the rank of those concerned. It may possibly mean other facilities for getting about, such as boots and shoes, or provisions: for any army’s march depends upon all these things.
Where people fight as volunteers for a cause, without an extensive war fund, those who can afford it provide such things for themselves, but those without means, yet anxious to serve, have to be left behind. Their disappointment is in proportion to their eagerness to serve.” 
 The Holy Quran, Arabic Text with English Translation, Commentary and comprehensive Introduction [Year 2002 Edition] by Maulana Muhammad Ali, page 420
 The Holy Qur’an – Arabic Text With English Translation & Short Commentary By Malik Ghulam Farid, Page 397
 The Meaning of The Noble Qur’an by Abdullah Yusuf Ali, page 122 – 123 http://www.ulc.org/wp-content/uploads/2012/10/English-Quran-With-Commentaries.pdf