Signing certificate request with certificate authority created in openssl

I created a certificate authority in Ubuntu using this command:

openssl req -config /etc/ssl/openssl.cnf -new -x509 -keyout ck.pem -out cacert.pem -days 365

Then I created a certificate request using:

openssl req -out C.csr -new -newkey rsa:2048 -nodes -keyout c_p.key

I would like to sign this request using the certificate authority I created. For that I executed:

openssl ca -config /etc/ssl/openssl.cnf -policy optional -out C.crt -infiles C.csr

But I get the error:

Using configuration from /etc/ssl/openssl.cnf unable to load CA
private key 140189274035872:error:0906D06C:PEM
routines:PEM_read_bio:no start line:pem_lib.c:703:Expecting: ANY
PRIVATE KEY

How can I get rid of this error? Will this command use the authority I defined above or maybe some default authority in the system?

EDIT:

For completeness I leave the steps that I had to do:

1. Create the relevant files:

mkdir demoCA
mkdir ./demoCA/newcerts
touch ./demoCA/index.txt

Also create a "serial" file in demoCA that contains 01 and a newline at the end.

2. Certify:

openssl ca -config /etc/ssl/openssl.cnf -cert cacert.pem -keyfile ck.pem -out C.crt -infiles C.csr