The error means that the tool that your’e using is trying to insert or delete value that conflicts with its parent value. Let me explain it easier.
For example the tool tries to insert some image name into media gallery for product with ID 999, but product with ID 999 doesn’t exist in the database.
Most likely it is one-time error as it shouldn’t remain.
In case it remains, and you cannot import any products, I can offer you to try another solution:
to import same products. There’s free trial available, so you’re welcome to check it for free.