A few other users reported this, check other questions; it is typically:
insufficient memory with a large product system model, and not using the fast libraries ->
- download the fast libraries (click on "you can make openLCA faster" in the welcome page)
- increase the memory suited to your computer and to the database and model (in preferences). 10GB is ok for many databases.
And then restart.
Good luck!
Andreas
edit, based on your comment: the direct calculation assumes a simple system, where every product comes only from one process, there is a warning that it may not work when this is violaed and a recommendation to check this condition before. I assume in your case, the condition is not met - please make then a product system first and then calculate this product system. Thank you.