Uncheck the secondary stacks in Stack Exposure, or write a group rule that forces the QB in whenever two teammates appear.
Stack Exposure. A primary stack includes the QB. A secondary stack doesn't. Secondary 2 means two players from the same team without their QB. Secondary 1-1 means two skill players from the same game on opposite teams, which most people are happy to keep. Uncheck every secondary stack, then re-check Secondary 1-1 if you want it.
Group rule. Open Rules, click Add New Rule, choose the group type, and pick Automatic. Then:
If at least two primary players are used at RB, WR, or TE, grouped by team
Then use exactly one secondary player on the same team at QB
The rule gives you more control than the checkbox. Add a stat requirement so it only applies to players under a projection threshold. Or save it as a manual rule, which creates one rule per team, and uncheck the teams where you're fine playing two skill players without the QB, such as a team with a very high total. Stack Exposure is faster. The rule is more flexible.
Related: How do I control which positions can be used in my stacks? · How do I exclude the stacks I don't want?