1) A typical DVD from a disc contains the structure that consists of *.IFO, *.BUP and *.VOB files.
Unlike the rest, *.VOB files contain the actual video data.
Sometimes due to file encoding or other problems, Movavi application does not recognize the DVD structure, one can try copying the VOB files only to a folder on one's hard drive.
Then, while adding the VOB files into Movavi Video Converter, the issue does not arise.
2) Please check that the DVD does not have any copy protection.
3) Double-check that the DVD file is not corrupted.