I think you might want to read up on how symbols and footprints are connected.
A bit of reading material from the FAQ:
What are symbols and footprints
How to assign a footprint to a symbol placed in eeschema
If you use cvpcb to assign footprints for all your placed symbols importing the netlist should work.
There are two options how it can fail. One is that the footprint lib has changed between assigning the footprint via cvpcb and importing the netlist.
The other option is that you did not use cvpcb to assign the footprint.
One option here is that the footprint is already assigned in the library. (fully specified symbol)
Here again you run into problems if the footprint libs changed since you assigned the footprint. Or if you got the symbol from a third party it might be required to also install some footprint lib.
If you manually entered the footprint into the placed symbols footprint field you might have used a wrong syntax. (it should be lib_name:footprint_name)