You haven’t demonstrated proof of funds.
This reason for rejection reads that “you have not provided proof of sufficient means of subsistence, for the duration of the intended stay or for the return to the country of origin or residence.” You must be able to prove that you have enough savings or access to funds to finance your trip—in its entirety—and for your return home.
The ‘means of subsistence’ vary depending on the country you are making your application to and the intended accommodations (hotel versus budget accommodation). This ranges from €14 per day when applying through the Latvian Embassy to €92 per day when applying via Switzerland’s.
How to Avoid This
There are several ways to demonstrate proof of funds for a Schengen visa. You must be able to present recent financial statements (within one month), a credit card statement with a sufficient available limit, traveler’s checks bearing your name, pay slips and proof of employment, or supporting documents that attest to a sponsor’s willingness and ability to cover your expenses.
Keep in mind that evidence of prepaid accommodation and transportation will reduce the amount required for subsistence.