Cause
There are circular references in the Bills of Material (BOM).
For example, while configuring BOM in Item A, if the same stock item is used and when you try to sync, the system refers to the stock item and when it finds the BOM has used the same stock item, then Tally does not proceed further.
Solution
Correct any circular references in the BOM, by removing the stock item which has been used in the same BOM, and then proceed with data synchronisation.
In case the volume of data is huge, it is advised to increase the virtual memory of the system.