The following error appears when generating CSR for the domain:
Error: Unable to create a CSR: openssl failed Internal error: Unable to generate data: MPC->encode() failed: openssl failed
Such behavior is known as software issue with ID #PPPM-1837.
The issue will be fixed in further updates of product.
As workaround set
OPENSSL_CONF system variable value to C:\Program Files (x86)\Parallels\Plesk\admin\conf\openssl.cnf as shown bellow:
setx -m OPENSSL_CONF "C:\Program Files (x86)\Parallels\Plesk\admin\conf\openssl.cnf"