"Please do not wait in line in front of the convenience store."
In my view, both「コンビニの前で--」and「コンビニの前に--」are OK in this sentense.
I said "de" instead of "ni" and got it wrong. It's pretty much the same and should be right.
Actually 前 (まえ) is nearly always used with に、and in this situation で would be wrong.
See my reason in reply to Nush_W. 前 (まえ) is not nearly always used with に. For example, off the top of my head, you would use で in the following case and never に: 僕の前でうんちしないで。
Doesn't this mean "Don't physically do it in front of me, in the spot that is literally right in front of me"? 僕の前にうんちしないで would mean "Don't do it while I am around"
'de' makes the area specific to in front of the convenience store, as opposed to somewhere else like the street or side of the store. I chose 'de' because someone saying please don't line up in front of the store is probably upset at you and telling you to move elsewhere. I can understand the use of 'ni' for 列に並ぶ using the intransitive argument, but I don't feel it is necessarily the reason for this sentence.