Hi @Add-in Developer ,
Thank you for posting in this community.
Ensure that the list has the appropriate permissions set so that all users can access it(can be used by all SharePoint websites). You can use the “Everyone except external users” group to grant read or edit permissions.
Here are steps:
1.Enable Everyone group in site level, Setting->Site information->Public - anyone in the organization can access this site.
2.List settings->Permissions for this List> Stop Inheriting Permissions> Grant Permissions> Enter the Everyone Except External Users group and select it> Show options> Select Edit/Read as permission level> Share.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.