In plain words
If you are unable to fix the mind steadily in Me, then seek to reach Me through abhyāsa-yoga, Dhananjaya.
Word for word
If you are not able to settle the mind steadily in Me, then by abhyāsa-yoga seek to reach Me, O Dhanañjaya.
अथ चित्तं समाधातुं न शक्नोषि मयि स्थिरम्अभ्यासयोगेन ततो मामिच्छाऽऽप्तुं धनञ्जय
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
अथ एवं यथा अवोचं तथा मयि चित्तं समाधातुं स्थापयितुं स्थिरम् अचलं न शक्नोषि चेत् , ततः पश्चात् अभ्यासयोगेन, चित्तस्य एकस्मिन् आलम्बने सर्वतः समाहृत्य पुनः पुनः स्थापनम् अभ्यासः, तत्पूर्वको योगः समाधानलक्षणः तेन अभ्यासयोगेन मां विश्वरूपम् इच्छ प्रार्थयस्व आप्तुं प्राप्तुं हे धनञ्जय ॥ ९ ॥
In plain words
And if you cannot do it — if you cannot settle your mind on me and hold it there, steady, unmoving, the way I have just described — then afterwards there is the yoga of practice. Practice means this: gathering the mind in from everywhere it has wandered, and setting it down on one single support, again and again. The yoga that grows out of that repetition, whose one mark is concentration — by that yoga of practice, wish to reach me, ask to reach me, the one whose form is everything there is, Dhanañjaya.
Close to the Sanskrit
Now, if in this way — as I have said, so — you are not able to concentrate, to establish, the mind on Me, steady and unmoving, then afterwards, by the yoga of practice: practice is the repeated placing of the mind upon one single support, having gathered it in from all sides; the yoga preceded by that, characterized by concentration — by that yoga of practice, wish, that is, pray, to obtain, to reach, Me who am of universal form, O Dhanañjaya.