Your account is connected to Facebook account, and your Facebook account's future off-Facebook activity is currently turned off.
If you get this error, it's due to a setting on your Facebook account called "Off-Facebook activity." According to Facebook, "Off-Facebook activity is a summary of activity that businesses and organizations share with us about your interactions with them, such as visiting their apps or websites." Essentially, this is needed for you to connect your Instagram account with mangosigns.com and display Instagram posts to your slides/signs.
You can visit this help article from Facebook to learn more about Off Facebook activity and how to enable it. You'll then want to turn it on if you want to use your Instagram or Facebook content on your signs.