Unfortunately, you did not send the specified document. You might have sent what you sent in good faith, and the ECO may have had no doubts that you had the funds required, but there are only limited number of documents you are allowed to provide. You chose to send electronic copies, which in order to be acceptable must be either stamped by the issuing bank, or accompanied by a letter from the bank verifying the authenticity of he statements. This information is stated both in the
Tier 1 General Policy Guidance and on the
UKBA Website for Maintenance Requirements.
As to how to proceed from here? Make a new application and send either a letter from your bank meeting the criteria in the linked documents, original bank statements, or the same type of electronic copies but stamped by your bank per the guidance. An Admin Review request would be useless as the ECO does not appeared to have erred in reaching their decision.
Many Immigration Consultants offer a document checking service.
Finally, you should be aware that the per Paragraph 202 of the Guidance:
All evidence must be dated no more than one month before the application is submitted.
rather than the old 7 day rule. With bank statements, only the most recent statement must meet the requirements of 202.