I am using google analytics data API. Making an API Call as listed here - https://developers.google.com/analytics/devguides/reporting/data/v1/quickstart-client-libraries.
My question is how do I enabled google analytics Data API for an already existing project. I need access to specific property ID.
any information is highly appreciated.
You have to follow Step 1 (Enable the API) and Step 2 (Add service account to the Google Analytics 4 property) in your link. Then use your property ID called Measurement ID (i.e. G-XXXXXXXX) that you can find in Data Streams (selecting a Data Stream) in Google Analytics 4 Admin Panel.
Following google support link, you should first log into analytics.google, then in admin page 
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then click on GA4 setup Assistant like the photo 
