TAN surrender application attachment error

It may be some temporary technical issue. 

I found below Extract from instruction to fill the FORM:

"Applicant may submit the TAN allotment letter received from Income Tax Department as a proof of TAN. In case the applicant does not have TAN allotment letter, he can submit a printout of TAN details using TAN search facility provided at www.incometaxindia.gov.in or www.tin-nsdl.com"

you may also try uploading the print out of TAN details using search facility.

Or else you may raise your query at Complain & Queries section of TIN-NSDL portal for any technical support.