In plain words
For the uncollected there is no buddhi, nor meditation; without meditation there is no peace, and for one without peace, how can there be happiness?
Word for word
There is no buddhi for the uncollected, nor for the uncollected is there meditation; for one who does not meditate there is no peace; for one without peace, whence happiness?
The commentators
Bhagavadgītābhāṣyam
सेयं प्रसन्नता स्तूयते —
नास्ति न विद्यते न भवतीत्यर्थः, बुद्धिः आत्मस्वरूपविषया अयुक्तस्य असमाहितान्तःकरणस्य । न च अस्ति अयुक्तस्य भावना आत्मज्ञानाभिनिवेशः । तथा — न च अस्ति अभावयतः आत्मज्ञानाभिनिवेशमकुर्वतः शान्तिः उपशमः । अशान्तस्य कुतः सुखम् ? इन्द्रियाणां हि विषयसेवातृष्णातः निवृत्तिर्या तत्सुखम् , न विषयविषया तृष्णा । दुःखमेव हि सा । न तृष्णायां सत्यां सुखस्य गन्धमात्रमप्युपपद्यते इत्यर्थः ॥ ६६ ॥
In plain words
Now this very tranquility is praised further —
'There is not' simply means: it does not exist. For someone undisciplined, whose mind lacks concentration, there is no understanding whose object is the true nature of the Self. And for that undisciplined person, there is no contemplation either — no real absorption in Self-knowledge. Likewise, for someone who never contemplates, who never gives himself to that absorption, there is no peace, no quiet settling. And for someone without peace, where would happiness even come from? Real happiness is simply this: the ending of the senses' craving to keep indulging in objects. Craving that is still aimed at objects is not happiness at all — it is nothing but suffering. As long as craving is present, not even the faintest whiff of real happiness is possible. That is the meaning.
Close to the Sanskrit
This very tranquility is [now] praised —
'There is not' — does not exist, is not: such is the meaning — [namely,] understanding whose object is the Self's own nature, for one who is not disciplined, whose mind is not composed. Nor, for one not disciplined, is there contemplation, absorption in Self-knowledge. Likewise, nor, for one not contemplating, not engaged in absorption in Self-knowledge, is there peace, quiescence. For one without peace, whence happiness? For the cessation of the senses' craving to indulge in objects — that is happiness; craving directed at objects is not [happiness]. That is indeed suffering. While craving is present, not even a mere whiff of happiness is possible: such is the meaning. [End of commentary on verse 66.]