Ordination procedure in Theravada Buddhism - What if parents are not alive anymore? I read that a person must get permission from his og her parents but what if that persons parents are not alive anymore. Can that person then not become ordained in this life?

Thank you for your time.
The commentary to this rule has some interesting discussion, including an answer to your question:

mātāpitaro matā, dārako cūḷamātādīnaṃ santike saṃvaddho, tasmiṃ pabbājiyamāne ñātakā kalahaṃ vā karonti, khiyyanti vā, tasmā vivādupacchedanatthaṃ āpucchitvāva pabbājetabbo. anāpucchā pabbājentassa pana āpatti natthi.
In the case where one's mother and father are dead, a child brought up in the home of a &quot;little-mother&quot; (step-mother? aunt?) etc., when causing them to go forth their relatives make a quarrel or feel dejected, therefore, for the purpose of cutting off dispute, should also be made to go forth after asking leave. But in the case of causing them to go forth without asking leave, there is no offence.
-- mahāvagga-aṭṭhakathā, 1. mahākhandhakaṃ, rāhulavatthukathā

The commentary then says something that is not quite clear to me:

putto attānaṃ nissāya jīvati, na mātāpitaro. sacepi rājā hoti, āpucchitvāva pabbājetabbo.
A son lives in dependence on himself, not on his mother or father. If he is a king, he should be made to go forth without having asked leave at all.
-- Ibid

I think these two sentences are meant to go together, and the period is a modern error, but it doesn't then say what a non-king should do in this instance. I don't see this as an issue, since clearly the previous statement still stands - without parents, asking permission is simply a courtesy.
Bonus - the commentary also contains the following edification:

eko mātāpitūhi saddhiṃ bhaṇḍitvā “pabbājetha man”ti āgacchati, “āpucchitvā ehī”ti ca vutto “nāhaṃ gacchāmi, sace maṃ na pabbājetha, vihāraṃ vā jhāpemi, satthena vā tumhe paharāmi, tumhākaṃ ñātakaupaṭṭhākānaṃ vā ārāmacchedanādīhi anatthaṃ uppādemi, rukkhā vā patitvā marāmi, coramajjhaṃ vā pavisāmi, desantaraṃ vā gacchāmī”ti vadati, taṃ jīvasseva rakkhaṇatthāya pabbājetuṃ vaṭṭati.
One, having quarrelled with their mother and father, comes saying &quot;cause me to go forth.&quot; And when told, &quot;having asked leave, come,&quot; says, &quot;I will not go. If you do not cause me to go forth, I will set the vihara on fire, or I will beat you with a stick, or I will bring about some unbeneficial thing like destroying the homes of your relatives and supporters, or I will fall from a tree and die, or enter into the midst of thieves, or go to the borderlands,&quot; it is proper, for the sole purpose of protecting life, to cause him to go forth.
-- Ibid