Failed to load resource: the server responded with a status of 403 (Forbidden)

I just installed the optin-monster plugin to create pop-form. I get this error and it is not able to load.Any help will be appreciated.

Here is the error i got.

Failed to load resource: the server responded with a status of 403 (Forbidden)

{readyState: 4, responseText: “
?<b>Notice</b>: Trying to get property of n…uld not be retrieved.”,”type”:”optin-data-error”}”, status: 403, statusText: “Forbidden

(a)
always: ()
complete: ()
done: ()
error: ()
fail: ()
arguments: null
caller: null
length: 0
name: “”
prototype: k.add
__proto__: ()
<function scope>
getAllResponseHeaders: ()
getResponseHeader: (a)
overrideMimeType: (a)
pipe: ()
progress: ()
promise: (a)
readyState: 4
responseText: “
?<b>Notice</b>: Trying to get property of non-object in <b>/dom9512/wp-content/plugins/api-functions/api-functions.php</b> on line <b>351</b>
?{“error”:”The optin ID for the user ID specified could not be retrieved.”,”type”:”optin-data-error”}”
setRequestHeader: (a,b)
state: ()
status: 403