There's no such thing as a service account. They're just another user account. You'll need to license it appropriately. Otherwise, for automation, always look to using Client ID + Certificate/Secret solutions rather than username/password, especially as you would not want to disable MFA on an elevated account.
Create a service account for SharePoint Online

Hi Guys,
Is there any way to create a "service account for SharePoint Online" we want to manage this account for running flows and other admin activities
Microsoft 365 and Office | SharePoint | For business | Windows
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Echo Du_MSFT 17,316 Reputation points
2021-02-04T09:10:06.007+00:00 Hello @SR VSP ,
In SharePoint Online environment, users don’t have the ability to manage SharePoint services as what they can in on premises environment, no such “Service Account” concept in SharePoint Online.
You could refer to this article to perform a SharePoint App-Only Authentication In Power Automate:
How To Perform A SharePoint App-Only Authentication In Power Automate
Thanks,
Echo Du====================
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