In Romans 15:20, Paul aimed to preach where Christ was not known, so as not to build on what?

Answer: Another man's foundation

Paul aspired to preach where Christ was not known, so he would not be building on someone else's (KJV: another man's) foundation.

“In this way I have aspired to preach the gospel where Christ was not known, so that I would not be building on someone else’s foundation.”— Romans 15:20 (BSB)