Could not install cart sms extension in opencart.2.0.1.1

I tried to install cart sms extension using extension installer but I get error message like

SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data
OK
<b>Warning</b>: ftp_login(): Sorry, cleartext sessions are not accepted on this server. in <b>/mystore.com/admin/controller/extension/installer.php</b> on line <b>295</b>{"error":"Could not login as mystore"}


But the FTP details are correct. I also tried installing Quick fix extension which is used for installing extensions without FTP. But that also doesn't work. It shows FTP needs to be enabled in the settings.

Thanks in advance.