Unable to send invoice to customer via email

Please help to solve below error.

wkhtmltopdf reported an error:
QSslSocket: cannot resolve CRYPTO_num_locks
QSslSocket: cannot resolve CRYPTO_set_id_callback
QSslSocket: cannot resolve CRYPTO_set_locking_callback
QSslSocket: cannot resolve sk_free
QSslSocket: cannot resolve sk_num
QSslSocket: cannot resolve sk_pop_free
QSslSocket: cannot resolve sk_value
QSslSocket: cannot resolve SSL_library_init
QSslSocket: cannot resolve SSL_load_error_strings
QSslSocket: cannot resolve SSLv3_client_method
QSslSocket: cannot resolve SSLv23_client_method
QSslSocket: cannot resolve SSLv3_server_method
QSslSocket: cannot resolve SSLv23_server_method
QSslSocket: cannot resolve X509_STORE_CTX_get_chain
QSslSocket: cannot resolve OPENSSL_add_all_algorithms_noconf
QSslSocket: cannot resolve OPENSSL_add_all_algorithms_conf
QSslSocket: cannot resolve SSLeay
QSslSocket: cannot call unresolved function CRYPTO_num_locks
QSslSocket: cannot call unresolved function CRYPTO_set_id_callback
QSslSocket: cannot call unresolved function CRYPTO_set_locking_callback
QSslSocket: cannot call unresolved function SSL_library_init
QSslSocket: cannot call unresolved function SSLv23_client_method
QSslSocket: cannot call unresolved function sk_num
QSslSocket: cannot call unresolved function SSLv23_client_method
QSslSocket: cannot call unresolved function SSL_library_init
QSslSocket: cannot call unresolved function SSLv23_client_method
QSslSocket: cannot call unresolved function SSL_library_init
QSslSocket: cannot call unresolved function SSLv23_client_method
QSslSocket: cannot call unresolved function SSL_library_init
QSslSocket: cannot call unresolved function SSLv23_client_method
QSslSocket: cannot call unresolved function SSL_library_init
Exit with code 1 due to network error: UnknownNetworkError
QSslSocket: cannot call unresolved function CRYPTO_num_locks
QSslSocket: cannot call unresolved function CRYPTO_set_id_callback
QSslSocket: cannot call unresolved function CRYPTO_set_locking_callback

Hi,

May we know the operating environment , version of frappe/ERPNext and how it was installed? Th errors suggest that SSL library may be missing , maybe this will help: https://github.com/amra/DocumentationAsCode/issues/2

What does: wkhtmltopdf -V , in a terminal return?