If you do not have a Microsoft O365/Outlook/Hotmail email address, you will need to follow these steps to associate your email address with a Microsoft account by using your email and following these instructions.
You will receive email notification from SharePoint Online (SPO) that the site has been shared with a link to the site.
Click on the Site Name to navigate to the SharePoint Online site.
Login to O365 account when prompted or if already logged into O365 account, you will get automatically logged into the SPO site.
If you try accessing this account without first verifying that your email is associated with a Microsoft account, you will have the below experience.
Guest navigated to sign in screen and enter email address.
You will receive this error message and will need to follow these instructions first before trying to sign in to access the SharePoint Online site.