Office 365 Global Administrator can configure external sharing at two levels:
· SharePoint Online environment (tenant);
· individual site collections.
To enable external sharing on the site collection level you should check whether the feature is enabled on BOTH tenant and site levels:
Configure external sharing for your SharePoint online environment:
1. Log into the Office 365 portal > Admin centers > SharePoint.
2. Click Sharing
3. Check whether external sharing is enabled.
Check external sharing for individual site collection:
1. Go to the SharePoint Admin center, click Active sites.
2. In the Site Collections list, select the specific site collection(s) you want to share.
3. Click Sharing to enable the external sharing and save the changes:
And now you have a possibility to share the site.
NOTE: If you decide to disable external sharing for individual site collection later, existing external users will not be able to access content inside the selected site collections. If you want to temporarily limit external user access, consider turning off external sharing from the Tenant Settings page instead.
For more information, please refer to this article.