When to choose SharePoint Online
Choose SharePoint, when your organization subscribes to that service in Microsoft 365 (Enterprise, Education, Government and DvNext). You'll enter your work or organization account to continue the sign-on process. Microsoft 365 Personal and Home subscriptions do not include SharePoint.
When to choose SharePoint Server
Choose SharePoint Server when your organization provides SharePoint Server 2019, SharePoint 2016 or SharePoint 2013, which have a different login process than SharePoint. You'll enter the web address of your SharePoint Server to continue the sign-on process. The web address, also known as a URL, may look like http://portal. This option is sometimes referred to as SharePoint On-Premises or On-Prem.
Note: It's likely that access to your SharePoint Server will be restricted by your organization. For example, you may not be able to reach it when you are not on the corporate network.
If you're still unsure which option to choose
If you're still unsure, ask someone at your organization if you have SharePoint in Microsoft 365 (Enterprise, Education, Government and DvNext). If not, ask for the web address of your SharePoint Server 2019, SharePoint 2016 or SharePoint 2013 server. If none of those options work, you may be able to identify the correct option from a web browser when logged in to SharePoint. You'll know you're using SharePoint when you see the words Microsoft 365 in the app launcher 
