If Contractor is responsible for design correctness as per contract and 70% of design drawings require correctness .Can he request for time extension?

Unable to extend the time for it must present valid schemas

I guess it depends....

What did the contractor base his design on? is is the specifications that are at fault or is there a conflict somewhere that was not spotted by the sales engineer at bid time.

Usually an RFI and further clarifications are needed to clear up any ambiguity and make sure the submittal and design drawings are per the customer's requirements.

If the contractor is at fault and did not do the due diligence, then they will have to catch up time lost and still meet critical milestones.