Matthew 5:37 NASB - 37 "But let your statement be, 'Yes, yes' or 'No, no'; anything beyond these is of evil.
It is okay to say you don't know if you don't know, or if you don't know something. You can say I don't know.
2 Corinthians 1:17-19 NASB - 17 Therefore, I was not vacillating when I intended to do this, was I? Or what I purpose, do I purpose according to the flesh, so that with me there will be yes, yes and no, no at the same time? 18 But as God is faithful, our word to you is not yes and no. 19 For the Son of God, Christ Jesus, who was preached among you by us--by me and Silvanus and Timothy--was not yes and no, but is yes in Him.
James 5:12 NASB - 12 But above all, my brethren, do not swear, either by heaven or by earth or with any other oath; but your yes is to be yes, and your no, no, so that you may not fall under judgment.