Does "a mechanism" mean 'some mechanism"?
"It's important in this type of system that a mechanism exists to allocate the scarce resources of our society, both natural resources and labor, in an efficient manner."

Question

Answer

The word 'some' can be used to describe an unknown or unspecified thing.
It can describe a single thing or plural things.
So, in your sentence, 'a mechanism' can be replaced by 'some mechanism'.
However, to avoid confusion, it is better to use 'a'.

For example (singular usage)
Some suggestion (a suggestion) would be better than no suggestion.
Some plan (a plan) is needed.