This indicates that you either have too many sub-BOM's or you have a BOM referencing loop.
Here's a pretty simple explanation: www.navisionplanet.com/.../
Sub-BOM's is when components on your current BOM have BOM's of their own. For example...
BOM for Item 1 has Item 2 on it. (Level 1)
BOM for Item 2 has Item 3 on it. (Level 2)
BOM for Item 3 has Item 4 on it. (Level 3)
... NAV has a maximum of 50 levels that it allows.
A BOM Loop would look like this...
BOM A has Item B on it.
BOM B has Item C on it.
BOM C has Item A on it.