In plain words
Those without buddhi think of Me, who was unmanifest, as having now come into manifestation, not knowing My supreme, imperishable, unsurpassed nature.
Word for word
The ones without buddhi think of Me, unmanifest [before], as come into manifestation, not knowing My supreme nature, imperishable, unsurpassed.
The commentators
Bhagavadgītābhāṣyam
किंनिमित्तं मामेव न प्रपद्यन्ते इत्युच्यते —
अव्यक्तम् अप्रकाशं व्यक्तिम् आपन्नं प्रकाशं गतम् इदानीं मन्यन्ते मां नित्यप्रसिद्धमीश्वरमपि सन्तम् अबुद्धयः अविवेकिनः परं भावं परमात्मस्वरूपम् अजानन्तः अविवेकिनः मम अव्ययं व्ययरहितम् अनुत्तमं निरतिशयं मदीयं भावमजानन्तः मन्यन्ते इत्यर्थः ॥ २४ ॥
In plain words
Why is it that they do not turn to Me alone? That is what is said next—
'Unmanifest' means not showing itself, not open to view. 'Come into manifestation' means come into plain sight. That is what they take me for now — though I am the Lord, eternally established, and always have been. 'Abuddhayaḥ': the undiscerning. 'My supreme reality' means the very nature of the supreme Self. Not knowing it, these undiscerning people — not knowing this reality of mine, which never wears away and which nothing surpasses — think of me that way. That is the sense.
Close to the Sanskrit
On account of what do they not resort to Me alone — thus it is said —
'Avyaktam' — unmanifest, not shining forth; 'vyaktim āpannam' — come into manifestation, gone to visibility: now they think me so, though I am the eternally established Lord, being such — 'abuddhayaḥ', that is, the undiscerning. 'Paraṃ bhāvam' — the very nature of the supreme Self — not knowing, the undiscerning; 'mama avyayam' — free from any wasting away; 'anuttamam' — unsurpassable; not knowing this being of mine, they think so. This is the meaning.