For posting to facebook’s page from the script you would need the long lived access token, normal access token will allow you to use it only once.
Following are the steps on how you can create the long lived access token?
1. Visit https://developers.facebook.com/tools/explorer/145634995501895/
2. Under application select the facebook application you want to use and make a note of App ID & Secret Key.
4. Get-> type in /<page_id>?fields=access_token and hit submit you will receive access token. Save this as we will use in next step.
5. Under graph API use oauth/access_token with POST method and following parameters
a) grant_type = fb_exchange_token
b) client_id = <app id>
c) client_secret = <app secret>
d) fb_exchange_token = <access token you just received>
Click submit and you should receive access_token which is long lived access token
For detailed visual steps you can view below video or visit video https://www.youtube.com/watch?v=xNFtjkLrQKM