The sign-in name or password does not match one in the Microsoft account system. (4376015)

1. A bad combination of Global Admin username or password was entered.
2. OneDrive not provisioned.
3. Customer environment has a Conditional Access Policy that blocks legacy authentication. Microsoft.Online.SharePoint.PowerShell module does not work with modern authentication in unattended mode.

Resolution 1

• Login to the On Demand Migration portal
• Click Migrate on the left
• Click the Projects link
• Locate and open your Project
• Go to Accounts > Dashboard
• Click Configure Connections
• Select the Target and click Edit to update the credentials
  • This account should not have MFA enabled
• Click Save
• Click Finish

Resolution 2

• Login to the On Demand Migration portal
• Click Tenants on the left
• Select the Source / Target tenants and click Edit Consents
• Regrant consents for Core/Basics and Migration modules
• Migrate OneDrive again to provision and migrate the target user(s) OneDrive 

Resolution 3

https://learn.microsoft.com/en-us/answers/questions/805576/connect-sposervice-the-sign-in-name-or-password-do

Workaround is either making an exclusion in the Conditional Access Policy that blocks legacy authentication for the user account or using the PnP module to connect to SharePoint Online.

Note: 

• To pre-provision OneDrive prior to migration, refer to https://learn.microsoft.com/en-us/sharepoint/pre-provision-accounts