Yōga Sūtra 2.24

तस्य हेतुरविद्या।

tasya hēturavidyā।

[तस्य हेतुः अविद्या।
tasya = its (non being in-unison); hētuḥ = causes; avidyā = is ignorance;]

Its cause (for not being in-unison) is ignorance (avidya).

Ignorance is the cause, because of which Jeeva has developed ego, and is unable to see both Iswara and Jeeva together.