That is a tough one without seeing a specific file, and for me, actually a pretty rare error.
The two main causes for me are. Bad geometry, i.e. something within TC has gone wrong and the facets making up the ACIS solid are broken or not being calculated correctly, this is the worse to sort out because there is not indication (prior to 2018) that something is wrong. Sat healing can sometimes (but rarely in my case) help, as can copy and paste into a new file, or if the part is symmetrical, slicing and mirroring the good half can work.
A second cause can be a previous Boolean that didn't go right. I recently added two parts together but was not careful enough aligning things up. therefore although the operation was successful (no error), it left a tiny 'crack' in the object, (about 0.00001 mm wide). When I came to add another part it failed, it took me a while to find and seal the 'crack', but after doing so, the new parts added OK.
Possibly the mort common problem for me, is aligning the objects, if possibly its best to overlap the objects, but obviously sometimes that is not possible, so taking care when aligning up edges of the objects can save future errors. When adding two parts where one would expect the edges to 'blend' together into one edge, and after adding, there is a line there the add has taken place. Do a quick check (zoom in) to see if it is just how TC is showing the object, or if the add produced a fault.
The above is just my thoughts. Others may have a different view, or found other problems with Boolean add.