Jeremiah 29:11 was a letter from God to the Jewish exiles in Babylon (more on that here). They were going to be stuck there for seventy years, and he was telling them he hadn’t forgotten them and would eventually bring them home.
God also told them to build houses, plant gardens, get married and pray for the city they were living in. Settle in and get on with life there.
So the promise came with a command to live faithfully in the meantime.