A available job.
A job available
-There are plenty of jobs available in the area.
-There are plenty of available jobs in the area.
Can we replace a noun with an adjectice?
I just would like to know whether the two sentences are OK or not.
They're both fine, but you asked "Can we replace a noun with an adjective?" In the examples you gave, where did you want to do this?
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.