This error is coming because you are opening the CS6 file in CS5.5 verison of Indesign ,
the one who has send you this file , tell him/ her to send you the IDML file after exporting from Indesign CS6
The IDML file will be compatible with the CS5.5.
Moving this discussion to the InDesign forum.
I've had the same message (but my version is CS4), if I ask the person who has sent the files to me, to resend them in IDML, can this also be useful for CS4?
You should be able to read .idml in CS4, but that doesn't mean the translation will be flawless. Any new features that have been added in CS5, 5.5, or 6, and that the original document uses will be lost, and the text will mostllikely flow a bit differntly due to changes in the text engines from version to version. If youare supposed to send the files back again for further editing in a later version, that will expose you to reflow as well.
You really should consider an upgrade if you need to collaborate with users of CS6.
I have CS5.5 and can't open a CS4 file (.indd) for the same reason. Window pops up stating I need to upgrade plugins to their latest version or get latest version of InDesign (disagree with having to do the latter). Confirmed file as CS4, not newer version CS6 and beyond.
Where can I go to acquire and upload these plugins? Your assistance is appreciated in advance.
@SilverCarMan – it's very unlikely that the supposed CS4 file is really a CS4 file.
You could check which version the file is you are trying to open.