Picture one piece of furniture: a big flat board, tilted towards the window, with a sheet of paper pinned down at the corners. A designer stands in front of it and the whole plan lives there as lines on paper, and nowhere else yet. From that one picture come the two expressions English uses far more often than the board itself. While the sheet is still pinned up, the thing is only being planned — it is on the drawing board. When the design fails, the designer walks back to the empty board with a fresh sheet — back to the drawing board.
From the picture
圖還在板上的那個階段（固定說法）
兩個意思都不是 drawing board 這個字單獨的意思，而是整句話的意思，只活在固定說法裡。它們共用同一個畫面：東西還在板子上，就還沒變成真的。差別只在介系詞——on 是「還在畫」，back to 是「重新畫」。
noun · B2
If a plan or a product is on the drawing board, it is still being planned or designed, and has not been built, made or started yet.
還在規劃中
noun · B1
A large flat board that an artist or a designer fixes paper to and works on, so that the paper stays flat and straight while they draw.
製圖板；畫板