The in and out point duration you have marked on the timeline is greater than the duration of the source material that you are trying to place there.
In other words, for example, you are trying to replace a 5 second clip of audio with a 4 second one.
Remove the out point on the timeline, then edit the new material to the timeline. There will be some residual audio from the original clip left on the timeline. Click on it to select the residual (unwanted) portion, and press the delete key.