Hi
1) Clause 7.1, 7.2 and 7.3 of Goa Real Estate (Regulation and Development) (Regulatory Authority Form of Annual Statement of Accounts and Annual Report) Rules, 2017 will be applicable in present case and the same can be assessed at https://rera.goa.gov.in/reraApp/resources/staticpage/RERA_Rule_345.pdf.
2) If the builder has asked you to take possession in compliance of Clause 7.2 (wherein the builder has asked you to take possession within 15 days from date of OC), then you are liable to pay maintenance from the expiry of 15 days from the date of receipt of notice.
3) Vide clause 7.3 of the aforementioned notice, if the allottee fails to take possession upon receiving a written intimation from the builder, then allottee shall be liable to pay maintenance charges as applicable including all Government rates, taxes, charges, interest on delay and all other outgoing and expenses of and incidental to the management and maintenance of the said Project and the building thereon
4) So, you should challenge the issue of Occupancy Certificate itself by way of filing a WRIT in high court given that the parking is incomplete and surprising how the authorities issued Occupancy certificate without completion of parking area.
Hope this information is useful.