You're probably using a sequence preset with four mono tracks, and your clip has stereo audio, therefore a new stereo track must be created to use it in the sequence. Create a new sequence preset that has the appropriate audio tracks and save it as a custom preset, so you can go back to it later.
Thanks Colin. You were right on the problem and solution.
You can tell the channel-count of an Audio Track by looking in the upper-right of the Track's Header. There are three icons: a single "speaker" (mono/i-channel), two "speakers" (2-channel/stereo) and 5.1 for 6-channel 5.1.
If you do not have the correct channel-count Track, PrPro will create one, with the proper channel-count, automatically, when you drag such a Clip to the Timeline.
PS - if you find that you use certain channel-count Assets often, you can set the appropriate number of those as the default. My default Sequence is 2 mono, 4 stereo and 1 5.1 w/ 5.1 Master. If I do not use all of those in that Sequence (do not use the 5.1 that much), I just Delete Unused Tracks. In the default arrangement, mono will be first, stereo next followed by 5.1. However, you can Add Tracks (and spot them to several locations), to get any order that you want. As my mono Tracks are often SFX, I might NOT create them in the New Sequence, but instead add them down at the bottom, keeping my stereo Tracks up at the top. Choices, choices, choices.