The ability to undertake pilgrimage, which is mentioned in the Ayah, is considered essentialin the duty of Al-Hajj is the following:

﴾And Hajj (pilgrimage to Makkah) to the House (Ka'bah) is a duty that mankind owes to Allah, those who can afford the expenses (for one's conveyance, provision and residence); and whoever disbelieves [i.e. denies Hajj (pilgrimage to Makkah), then he is a disbeliever of Allah], then Allah stands not in need of any of the 'Alamin (mankind and jinns).﴿

[Aal-Imran,97]

  Thus, it is a physical, financial, security and administrative ability. A Muslim whose body cannot bear the hardships involved in performing the rites of Al-Hajj or is so pronounced by a physician who is a specialist, skillful and a pious Muslim, could have someone perform Hajj on his behalf during his lifetime, or he may entrust to someone to perform pilgrimage after his death, pursuant to the provisions of this type of pilgrimage.
  Allah does not consider a Muslim who does not have enough money to cover the types of travel expenses, the accommodation expenses in Mecca and Medina, the cost of food and drink, as well as the expenses of his family during his absence, able to undertake the pilgrimage. Hence, an unable Muslim should not humiliate himself in order to raise money for the Hajj expenses, nor should he resort to untoward ways and means, for he is not able and is exempted from performing the duty of Hajj.
 When officials in charge in Islamic countries resolve to adopt a certain system that enables those who cannot afford to perform Hajj to do so, and preclude those who have performed the Hajj duty not to go to Hajj again before five years have passed, when the motive of such regulation is to enable those Muslims who have not performed the Hajj duty to do so easily and peacefully, one must not commit a sin in order to perform a voluntary pilgrimage, as the Muslim who is not allowed to perform pilgrimage is not considered able to do so.