1) Make sure the discs you are using fit the correct format specified by your Unit . For example - if you are using DVD-R's that you have burned yourself and the unit does not support this format you will get this error.
2) Dirty laser lense causing the unit to have problems in reading the disk. Use a cleaning Disk like this one to see if the error resolves.
If the error remains and you feel confident to do so you can open the unit and look for the glass laser lense - usually blue in color under the DVD clamp. Use a cotton swab and some isopropyl alcohol to gently wipe the lens in order to clean it.
Allow to dry naturally for a couple of minutes before re-assembling the cover on the unit.
3) Try a CD in the unit - if the unit reads CDs and not DVDs - this is an indicator of a faulty or misaligned drive mechaninsm within the unit itself. In this case the drive unit will require service attention and need to be replaced.
Obviously you should check that any disks you are using are clean and free from scratches.
Hope this helps. Please do rate my answer. Many Thanks.