No. A seller who cannot find the existing survey isn’t exempt from furnishing it. If the seller agreed to deliver the existing survey to the buyer, he is required to deliver the survey and the affidavit within the specified time.
Paragraph 6C(1) says, in bold, "If Seller fails to furnish the existing survey or affidavit within the time prescribed, Buyer shall obtain a new survey at Seller's expense no later than 3 days prior to Closing Date." This means the seller will be responsible for the cost of a new survey if he can’t find the existing survey. Any party to the contract who doesn’t perform a "shall" obligation under the contract would probably be found in default by a court unless otherwise excused from performance by the terms of the contract.
The seller could try to obtain another copy from the surveyor or title company he used when purchasing the property so that he can fulfill his contractual obligations. To avoid this situation, sellers should only agree to provide an existing survey if they have it readily available.