"Never say maybe, because the Bible says let your yes be yes and your no be no." I've heard this claim in some Christian households, but the reality is the verse in question has nothing to do with absolute certainty. Rather, it is quite the opposite. To let your yes be yes and your no be no is about consistency, while avoiding taking what rightfully belongs to God.
When Jesus speaks about this concept in Matthew 5:34-37, He tells us that we must not swear by the Lord's throne (Heaven), His footstool (Earth), the city of the Great King (Jerusalem), or even a hair on your head, over which you have little control. The idea is that by swearing on anything, you are assuming a greater authority than the Lord Himself. Thus, to swear is blasphemy from Satan.
But what about a maybe? Would this not contradict a yes being yes and a no being no? Far from it: Letting your yes be yes and your no be no implies that you have given a yes or a no to begin with. Yes, I will go to your football game. No, your bum does not look big in this. I swear by... stop. If you have said yes to the game, let that yes be yes. If you have said no to your friend's fashion choice exaggerating a certain feature, let that no be no. You do not need to add an oath.
But there is precedent for saying maybe. In James 4:13-17, James criticises the folly of giving such a solid yes in the first place. Why? Because you do not know what will happen tomorrow (v14). Yes, I will go to your football game next week. That is, unless I get hit by a bus, or I get arrested, or I get called away on an important job, or my friend is seriously injured and I have to take them to the hospital, or... you get the picture. Anything from the tragic to the simple can hinder your yes. Therefore, even your most sincere yes should be, as James says, "If the Lord wills, we shall live and do this or that." Both your yes and your no should depend entirely upon the will of God, and with that in mind, you should actually always allow your yes and your no to be as conditional as a maybe.