B.G 2.66 and 67

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

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

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.

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

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.

"śāntirmōkṣōtha nirvāṇam।"

ityabhidhānāt ॥66॥

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

states the dictionary.

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॥

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.