Autoplace - how to use it

The layout editor requires that you create a placement grid for the area where you run any mode of automatic placement. Automatic placement places packages only at the intersections of the placement grid lines. I believe the due to this you must be getting some lack of place grids error that you're mentioning here.
Below link covers this in detail-
support.cadence.com/.../techpubDocViewerPage

Can you please share the exact error message that you're getting while using Autoplace option or a snapshot of it.
Also, while using Place> Autoplace, which is the mode you're using to run automatic placement here. 

I hope this is the option you're using to autoplace in allegro, else please do let me know if you're using something else which I may have misunderstood here. 

Thanks!
I am trying the "design" option first, as that seems like the least complicated.  Thanks for the link.  Their documentation doesn't really seem to get into the specifics of things like the place grid, which seem important for making the thing work.  I have a top and bottom package keeping shape that takes up the whole board, and I then make "Top Grids" and "Bottom Grids" settings and click on the keeping areas (top and bottom respectively) and it seem to make the grid.  I have tried many different sizes.  It sure seems like there is a placement grid.  Here is the error I get (I have also tried placing all the components first, but that doesn't work either):

Algorithm : IC
Place within: board
Map grid points to: library origin
Place leftovers on bottom: YES
Allow component overlap: NO
Soft boundary: NO
Cluster: YES
No Rat: NO
Remove TAG: NO
Components at rotation of 0 degrees are weighted 50

Components at rotation of 90 degrees are weighted 0

Components at rotation of 180 degrees are weighted 0

Components at rotation of 270 degrees are weighted 0

Board direction of north is weighted 50

Board direction of east is weighted 50

Board direction of south is weighted 50

Board direction of west is weighted 50

Component mirroring weight is 50

Component straightness weight is 50

Warning: unplaced components are not PLACE TAGGED.

Placement Keepin

line seg (700.00,16700.00) (700.00,500.00)
line seg (700.00,500.00) (20100.00,500.00)
line seg (20100.00,500.00) (20100.00,16700.00)
line seg (20100.00,16700.00) (700.00,16700.00)

Error: No Placement Grid was found

Error: See message(s) above

I found the problem.  Hitting "done" after setting up the placement grids breaks the whole process.  If I go through with the rest of the placement without hitting "done" after setting up the grids, it sometimes works (about 50% of the time it just crashes instead).

Thanks again for the info.  I wish Cadence would provide more specific documentation about this sort of thing that doesn't skip many important steps (how to "tag" the components for placement, for example, and the importance of not selecting done at the end of setting up the grids).

Check Youtube for an overview of a plugin called CircuitSpace from EMA-EDA. 

For Orcad users, it's called PCB Clustering.  

Highly recommended.   Layer mapping to dissimilar stackup/layer counts and opposite sides.

Can't imagine using Allegro without it.

Something that 99% of the Allegro users are not aware of is that the 'route editor' (i.e. specctra ) has a placement mode that is IMHO way better than the Allegro autoplace functionality