Quatrain 20

گیرم تو به ادراک معمّا نرسی

در نکتۀ زیرکان دانا نرسی

اینجا ز می لعل بهشتی می ساز

کانجا که بهشت است رسی یا نرسی

source, Dashti 49, p. 253

giram to be edraak-e mo'ammaa narasi
dar nokte-ye zirakaan-e daanaa narasi
injaa ze may-e la'l beheshti mi saaz
kaanjaa ke behesht ast rasi yaa narasi

Granted you will not solve the great riddle,
(you won't solve it --
not even if you use the subtle arguments
used by pundits and problem-solvers).
So right here
build a heaven of ruby-red wine,
that Heaven there
you may reach --
or just may not.

 O heart! that riddle thou wilt never read,
That point on which the wise are not agreed;
Quaff wine, and make thy heaven here below,
Who knows if heaven above will be thy meed? 
Whinfield, quatrain 427

Translation & discussion of the quatrain:
The speaker acknowledges that the only control is over the 'now';  'you' can't solve the riddle of existence and you can't be sure where you will end up once life ends.  Another well-known bit of advice against the not-trying-to solve this riddle and what is more important to do is found in Hafez ghazal 3 (in both the Qasvini-Ghani and Khanlari editions):

حدیث از مطرب و می گو و راز دهر کمتر جو
که کس نگشود  و نگشاید بحکمت این معما را

 hadis az motreb o may gu wo raaz-e dahr kamtar ju
ke kas nagshud o nagshaayad behekmat 'in mo'ammaa raa 

Talk about the music-player and about wine
and search only  a little for the secret of life;
no one has solved or ever will  solve this mystery in metaphysics.

1&2 -- I concede/granted/ that you will not arrive at the understanding of the riddle/you will not reach (it) through the subtlety of the learned problem-penetrators... giram, 'granting,' 'allowing, 'given' - 1st person singular, present/future tense, used adverbially as a (suppositional) qualifier. Hedayat and Forughi read ay del (above), 'O heart.'    Why Does Dashti have giram? Is it because his source, and I assume it his source, the collection Nuzhat al- Majaalis, has giram? What sources then are Hedayat and Forughi consulting?  A mystery I can't solve in palaeography or textual criticism.   zirakaan: it seems to me that this means 'those who get to the core of things/those who penetrate problems.'   zirakaan daanaa:  I believe these two adjectives would 'normally' be linked by the conjunction va/Englishand.  The repetition of words of similar meaning here underscores the irony (and it seems stronger to me without va )-- the speaker of this quatrain has said and will say that no one has penetrated (or is likely to penetrate) the mystery of existence and the universe -- see weblog quatrain 8, in bahr-e wojud and:

آنانکه محیط  فضل و آداب  شدند

در جمع کمال شمع اصحاب شدند

ره زین شب تاریک نبردند برون

  گفتند فسانه ای و در خواب شدند

Dashti, quatrain 3, p. 244

aanaanke mohit-e fazl o aadaab shodand
dar jam'e kamaal sham'e ashaab shodand
rah zin shab-e taarik nabordand berun
goftand fasaane'i yo dar khaab shodand 

The saint and seer profound in wit and lore,
Who torch of knowledge 'mongst the sages bore,
Out of the darksome night could find no way;
Some tales they told, then went to sleep e'ermore. 
Saidi, quatrain 57 

Kasra translates line 2: ...thou will not grasp the wit of the wise and clever ones. I think that the second line repeats the idea contained in the first line, 'you will not solve the puzzle by the subtle distinctions, etc. ' However, Kasra may be right if the verb is dar ... rasidan, and this 'split' would be an example of tmesis, which literally means a 'cutting' -- usually denoting the separation, primarily in poetry, of the elements of compound verbs .  I suspect that this device is rare in Khayyam, though it may have been used in quatrain 4, ... fakandash andar ....  Elwell-Sutton has  These problems you will never understand,/So leave them to the subtle men of science ...
3&4 -- Here build a paradise of ruby wine/since there, which is paradise, you may or may not reach.