Before installing Stackposts, please make sure following PHP requirements and extensions are installed and enabled on your server:

PHP version PHP version 5.6 ~ 7.1
allow_url_fopen allow_url_fopen=On (php.ini file)
PHP cURL Required. (Curl Library, Curl_init Function and Curl_exec)
PHP OpenSSL Required. (For secure data encryption.)
PHP PDO Required. (To create secure connection to MySQL server)
GD Required. (For image processing).
PHP EXIF Required. (For validating JPEG files)
PHP mbstring Required.
Cronjobs Required (The cronjobs must be configured for publising scheduled posts.)
FFMPEG Required for video posts. (For Video post types.)
Zip PHP zip extension is required (for updating, installing etc)
Option (Recommended) To upload large videos and some thing like that, you need to increase the following values in your server: 
memory_limit = 256M 
upload_max_size = 64M 
post_max_size = 64M 
upload_max_filesize = 64M
max_execution_time = 60
max_input_time = 500
max_connections = 200
max_user_connections = 200
wait_timeout = 180
max_allowed_packet = 512M