Failed to open storage in SOAP Project with Certificates

When I execute the definition of my SOAP service that requires a digital certificate I get the following error "Failed to open storage". This why it can happen?

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
<s:Body>
<SignPDFServerResponse xmlns="http://tempuri.org/">
<SignPDFServerResult>Failed to open storage</SignPDFServerResult>
</SignPDFServerResponse>
</s:Body>
</s:Envelope>

Thank you