The Rehearsal function automatically adjusts the Input Level so that the incoming audio level (such as band rehearsal, live performances, etc) will be -6dBFS while rehearsing. If this level cannot be acheived, a message shows "Error, Cannot Adjust".
If you're using the Rehearsal Function and receiving this error message, then the incoming audio is resulting in a level above -6dBFS in the R-07. In this case, it would be best to not use Rehearsal mode and try lowering the Input Level 
as much as needed so that clipping does not occur.
It's also recommended to turn the Limiter ON to further avoid clipping:
- Press the SCENE button.
- Cursor to "Limiter" and then use the left/right cursor arrows to set this to "ON."
