CPQ 'Access Declined' error in the Quote Line Editor

Check the following:

1. Insufficient permission to CPQ Objects. Quote, Quote Line, Quote Line Group at least. To check the proper permission sets, review the Get Started with Salesforce CPQ Permissions article.
2. Incompatible Price Rules. Disable all Price Rules running in the org and check again. You may have formulas in a Price Rule that are incompatible with the Advance Calculator. If deactivating a Price Rules removes the error, proceed in reviewing your Price Rules. For more information, read Write calculator compatible formulas for the Salesforce CPQ Advanced Calculator.
3. Browser-specific settings. Compare the behavior in Chrome and Firefox. This could be browser-specific.
   1. If you are getting the error in Firefox, this is most likely due to Mozilla Firefox’s Content Blocking feature, specifically the setting for Trackers (content, cookies, or scripts that collect browsing data) which is by default blocked in Private Windows, and if Content Blocking is set to Strict, Trackers may be blocked in all Windows. Please follow the below steps:
      • Navigate to Preferences
      • Select the Privacy and Security Tab
      • Select the Custom Content Blocking
      • Uncheck the Trackers checkbox to allow Tracking in all windows
4. Salesforce Organization Edition. Navigate to Company Information | Organization Edition. If you have Professional Edition and your Salesforce CPQ version is newer than Summer '17 (v26.x), you need to purchase an API license. The API Enabled permission can be granted to Profiles under System Permissions. If you are a Professional Edition user, please Set Up Salesforce CPQ in Salesforce Professional Edition as API access needs to be enabled for Salesforce CPQ.
5. The user does not have API Access. The API Enabled permission can be granted to Profiles under System Permissions.  
6. API Request 24-Hour Limit. If any of the information above has been checked, you may have reached the amount of API that can be sent within 24 hours. Salesforce has a governor limit on the number of API requests that can be sent within the org based on the Salesforce Edition and type of environment. Whenever Line Editor is opened, API requests are sent out in order to load the Quote information into the Visualforce page. Navigate to Setup | System Overview to check if you reached the maximum threshold.

If the issue cannot be resolved after investigating the details above, please log a ticket with Support.