B.G 2.66 and 67

नास्ति बुदि्धरयुक्तस्य न चायुक्तस्य भावना। न चाभावयतः शान्तिरशान्तस्य कुतः सुखम्॥६६॥

इंद्रियाणां हि चरतां यन्मनोनुविधीयते। तदस्य हरति प्रज्ञां वायुर्नावमिवाम्भसि ॥६७॥

nāsti budidharayuktasya na cāyuktasya bhāvanā। na cābhāvayataḥ śāntiraśāntasya kutaḥ sukham॥66॥

iṁdriyāṇāṁ hi caratāṁ yanmanōnuvidhīyatē। tadasya harati prajñāṁ vāyurnāvamivāmbhasi ॥67॥

[नास्ति बुद्धिः अयुक्तस्य न च युक्तस्य भावना। न च अभावयतः शान्तिः अशान्तस्य कुतः सुखम्॥
nāsti = does not exist; buddhiḥ = wisdom; ayuktasya = without gracefully engaged mind; na = not; ca = and; yuktasya = gracefully engaged; bhāvanā = attitude;। na = not; ca = and; abhāvayataḥ = who is short of; śāntiḥ = peace, liberation; aśāntasya = without peace, or liberation; kutaḥ = how can there be; sukham = happiness;]

[इंद्रियाणां हि चरतां यत् मनः अनुविधीयते। तत् अस्य हरति प्रज्ञां वायुः नावम् इव अम्भसि॥
iṁdriyāṇāṁ = of the senes; hi = certainly; caratāṁ = moves; yat = that; manaḥ = mind; anuvidhīyatē = subservient to; tat = from; asya = it; harati = steals; prajñāṁ = wisdom; vāyuḥ = windy; nāvam = ship is; iva = the way; ambhasi = waters]

For one who is without the gracefully engaged mind, there is no wisdom. Without the graceful attitude, one does not get liberated. Without liberation, where is happiness? Such a mind (without grace) is sub-servient to the movement of senses. Wisdom is stolen from such a mind the way the windy waters carry away a ship.

Gīta Bhāshya 2.66 and 67

Restraining the mind faculty is not suitable for the ungraceful mind. 'anu vidhīyatē'- lord instruments as per one's senses. 'harati prajñāṁ' - arising of wisdom from knowledge is prevented and even the existing wisdom shrinks.

प्रसादाभावे दोषमाह उत्तराभ्यां श्लोकाभ्याम्। न हि प्रसादाभावे युक्तिः चित्तनिरोधः। अयुक्तस्य च बुद्धिः सम्यग्ज्ञानं नास्ति। तदेव उपपादयति न चायुक्तस्येति। शान्तिः मुक्तिः।

prasādābhāvē dōṣamāha uttarābhyāṁ ślōkābhyām। na hi prasādābhāvē yuktiḥ cittanirōdhaḥ। ayuktasya ca buddhiḥ samyagjñānaṁ nāsti। tadēva upapādayati na cāyuktasyēti। śāntiḥ muktiḥ।

[prasādābhāvē = not having a serene mind; dōṣamāha = defect is said; uttarābhyāṁ = next; ślōkābhyām = two verses; na = without; hi = certainly; prasādābhāvē = not having serene, graceful mind; yuktiḥ = is suitable; cittanirōdhaḥ = for restraining the mind faculty; ayuktasya = without suitable (mind); ca = and; buddhiḥ = the intellect; samyagjñānaṁ = perfected; nāsti = does not become; tadēva = therefore; upapādayati = it is ascertained; na cāyuktasyēti = 'na cāyukta' verse; śāntiḥ = 'śānti' means; muktiḥ = liberation;]

Defect resulting from not having a serene mind is described in the next two verses. 'na yukta' means nongraceful mind, and because of which it cannot restrain the mind faculty. With such an unsuitable intellect, it is not possible to get perfected knowledge. Therefore, it is ascertained 'na cāyukta', i.e. 'it is not suitable' in the verse.
'shānti' means liberation.

"शान्तिर्मोक्षोथ निर्वाणम्।"

इत्यभिधानात् ॥६६॥

"śāntirmōkṣōtha nirvāṇam।"

ityabhidhānāt ॥66॥

["śāntirmōkṣōtha = 'Shanti', 'Mōksha'; nirvāṇam = 'Nirvāṇa" are synonymous,]

[ityabhidhānāt = states the dictionary;]

'Shanti', 'Mōksha' and 'Nirvāṇa" are synonymous,

states the dictionary.

कथं अयुक्तस्य भावना न भवति। आह - इंद्रियाणामिति। अनु विधीयते क्रियते ननु ईश्वरेण इद्रियाणाम् अनु। "बुदि्धर्ज्ञानम्'' इति वक्ष्यमाणत्वात् । प्रज्ञां ज्ञानम् उत्पत्स्यदपि निवारयतीत्यर्थः। उत्पन्नस्यापि अभिभवो भवति ॥६७॥

kathaṁ ayuktasya bhāvanā na bhavati। āha - iṁdriyāṇāmiti। anu vidhīyatē kriyatē nanu īśvarēṇa idriyāṇām anu। "budidharjñānam'' iti vakṣyamāṇatvāt । prajñāṁ jñānam utpatsyadapi nivārayatītyarthaḥ। utpannasyāpi abhibhavō bhavati ॥67॥

[kathaṁ = Why; ayuktasya = one without graceful; bhāvanā = attitude; na = not; bhavati = becomes (serene) āha = is said; - iṁdriyāṇāmiti = in the 'iṁdriyāṇām' verse; anu vidhīyatē = 'anu vidhīyatē' means; kriyatē = is instrumented; nanu = accordingly; īśvarēṇa = by the lord; idriyāṇām = the senses; anu = as per; "budidharjñānam" iti vakṣyamāṇatvāt = therefore it is said "budidharjñānam" in the verse (10.4, i.e wisdom, knowledge other faculties of the living beings are controlled by Me); । prajñāṁ = wisdom; jñānam = from knowledge; utpatsyadapi = even arising; nivārayatītyarthaḥ = prevents; utpannasyāpi = even the aleady produced; abhibhavō = reduced; bhavati = becomes;]

Why one without graceful attitude does not obtain (serenity)? This question is answered by 'iṁdriyāṇām' verse.
'anu vidhīyatē' means lord instruments as per one's senses. Therefore, it is said "Wisdom, knowledge and other faculties of living beings are controlled by Me" in 10.4 verse.
'harati prajñāṁ' means arising of wisdom from knowledge is prevented and even the already produced wisdom shrinks.