When you choose [File] > [Export Project] > [AAF], you can choose five presets of AAF style. If placed clips frame rates mismatch to project settings and you choose Type 3, this message may appear. Because this style does not allow to have different frame rate between timeline and placed clips.
If you choose other presets (Type 1,2, 4 and 5), placed clips will be exported with frame rate conversion to be matched to the timeline.

If you need to use Type 3 preset, follow below steps:

1. In the Bin, open clip properties (do NOT place clips onto the timeline)

2. Select Video > Framerate 

3. Select same framerate value as project settings

4. Click [Apply] then [OK] buttion

5. Place the clip onto the timeline