and sons of God see the daughters of men that they `are' fair, and they take to themselves women of all whom they have chosen. [YLT Genesis 6:2]
And Jehovah saith,
My Spirit doth not strive in man -- to the age; in their erring they are' flesh:' and his days have been an hundred and twenty years. [YLT Genesis 6:3]
Robert Young seems to be adding the verb 'erring' in interpretation, or explanation.
It appears to me that he is relating this text to Jude 1:6 :
And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day. [KJV]
I would suggest that Young is referring 'My Spirit' and 'striving' to the angelic powers who, unlawfully and inappropriately, manifested themselves in humanity ('left their first estate') in order to participate in the material creation, with catastrophic results.
'In their erring they are flesh' says Young, that is to say in their error of supposing they can, or should, leave their proper place and partake of flesh and blood.
'My spirit' that is not to say the Holy Spirit, but the angelic spirits, shall not 'strive' (unlawfully forcing themselves into an estate to which they do not belong) 'unto the age', that is to say, indefinitely.
Yet their days shall be an hundred and twenty years, that is to say, judgment will fall after a set time (once an ark is prepared).