I faced the same problem today, have solved it as follows (I am using the LAMP template in the list of one-click apps):
I. Installed the source code of CURL: wget https://curl.haxx.se/download/curl-7.60.0.zip
II. Installed the tools that are necessary to compile CURL: apt-get -y install binutils make gcc
III. Unzipped the file with CURL: unzip curl-7.60.0.zip.
IV. Went to the CURL directory: cd curl-7.60.0
V. Ran these commands:
./configure –with-nghttp2 –prefix=/usr/local –with-ssl=/usr/local/ssl
sudo make install
VI. Created a certificate authority (important, as the WHMCS error won’t go away until this and below steps are done):
openssl genrsa -out cert.key 2048
openssl req -x509 -new -nodes -key cert.key -sha256 -days 3650 -out cert.pem
VII. Opened the configuration file /etc/php/7.0/apache2/php.ini and edited two lines there, as follows:
curl.cainfo = /usr/local/ssl
VIII. Installed the php-xml module: apt-get -y install php-xml
IX. Restarted the Apache service: service apache2 restart
X. Rechecked the requirements on the WHMCS installation webpage, and both errors are gone.
I am sure there are easier ways to achieve this, but the trial and error resulted in the steps above, which worked for me. I invite others to provide better instructions (Googling for such didn’t yield anything so far).