Finding the ungrammatical word

I made the following question, and students have to find the word ungrammatical.

Would the underlined be ungrammatical? Originally it was The one thing that unites you all,~.

I was thinking "but" is a coordinate conjunction, therefore it could be ungrammatical.

However, I found from the dictionary "but" is also an adverb. Then, is below a correct sentence?

What if I change to "which" to be ungrammatical? (however I don't tp change much. There's much paperwork needed.)

Q: Is there anything special about teenage stress, compared with that of adults?

A: I believe so because growing up is not easy. Teenage experiences vary, and everyone experiences different emotions and concerns. The one thing but unites you all, however, is that you are changing, from being a youth to becoming an independent adult. During this time of transition you live with a lot of uncertainties and imbalances, which create huge anxieties. It is important to know that it is natural for you to feel stressed, and that it is okay. Just keep in mind that whatever stresses you out now will eventually pass.