It is necessary to connect your Instagram account with your website.
To do this click WP Dashboard => Instagram Feed => "Add Source":
https://i.imgur.com/1wknH15.png
Then click "Add Account Manually": https://i.imgur.com/D6QOm9T.png
Input your ID and Token in appropriate fields: https://i.imgur.com/AXlRxQY.png
Click "Add" and "Save changes"
Then check results at front-end.
You also find all necessary information about how to fix this issue via this link:
https://smashballoon.com/doc/instagram-api-error-message-reference/?instagram#999