During the public folders synchronization job creation wizard, on "Select Public Folders" page, all the source servers' public folders are grey out
The selected source public folder store is not a Default Public Store of the mailbox store where the source's administrator mailbox located
Please change the Default Public Folder Store of the mailbox store where source's administrator mailbox located to the selected source's public folder store.
1. Go to Exchange System Manager.
2. Locate the mailbox store.
3. Right click and select Properties.
3. Under General | Default public store, click Browse.
4. Click the selected Public Folder Store.
5. Click OK.