B.G 2.66 and 67

nāsti buddhiḥ ayuktasya na ca ayuktasya bhāvanā। na ca abhāvayataḥ śāntiḥ aśāntasya kutaḥ sukham॥66॥

indriyāṇāṃ hi caratāṃ yanmano'nuvidhīyate। tadasya harati prajñāṃ 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.

prasādābhāve doṣamāha uttarābhyāṃ ślokābhyām। na hi prasādābhāve yuktiḥ cittanirodhaḥ। ayuktasya ca buddhiḥ samyagjñānaṃ nāsti। tadeva upapādayati na cāyuktasyeti। 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.

"śāntir mokṣotha nirvāṇam।"

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

ityabhidhānāt ॥66॥

states the dictionary.

kathaṁ ayuktasya bhāvanā na bhavati। āha - indriyāṇāmiti। anu vidhīyate kriyate nanu īśvareṇa idriyāṇām anu। "bud'dhirjñānam'' iti vakṣyamāṇatvāt। prajñāṁ jñānam utpatsyadapi nivārayatītyarthaḥ। utpannasyāpi abhibhavo 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.

Gīta Tātparya 2.66 and 67

'shānti' - also means honest endeavour towards the lord.

śāntiḥ bhagavanniṣṭhā।

'śāntiḥ' means honest endeavour towards the lord.

"śamo manniṣṭhatā।"

"The root 'sha' means sincerity of the mind"

iti hi bhāgavate ॥64-66॥

-so stated in Bhagavata.