Unless the shield or the case is non standard, things should install.
First, be certain that the shield is oriented to match the motherboard rear panel connectors.
The shield is a press fit.
You need to press the shield into place a bit at a time going around the periphery.
All edges must be seated.
If this is really not possible, then there is a manufacturing error and you can abandon the shield.
Assuming the shield fits in place, you really need three hands to get the motherboard into place.
A helper is useful here.
Usually, you need to press the motherboard into the shield and find a way to hold it there. Takes one hand for that.
Some shields have spring like tabs that need to be compressed.
Next thread a motherboard post screw into one of the posts near the shield.
Leave it loose. This takes two hands. One to hold the screw in place, and another to turn the screwdriver.
If you have a magnetic tip #2 philips screwdriver to hold the screw this can be done with one hand.
Once that first screw is in place, you should be able to lever the motherboard against whatever spring resistance the shield offers.
Get all motherboard screws in loosely before securing them.
What is the make/model of your case, and parts inside?
Most of the time, two front intake fans of 120/140mm size and a 120mm rear exhaust
will be sufficient for an air cooler.
If all the air intake is from one source, and is filtered, your parts will stay relatively clear of dust.
Adding extra top fans will tend to draw in unfiltered air from adjacent openings,
It will also divert cooling air from the motherboard and cooler and out the top without doing any cooling.
AIO mounts get complicated and compromises must be made.