When migrating Sharepoint, there's a bit of a trick to it. How to perform the migration depends on whether that Sharepoint site is also a M365 Group. You can determine that in the Admin Portal under 'Groups' | 'Active Groups', like in the attached screenshot.
If the Sharepoint Site is listed there, then it is not just a plain Sharepoint site, it is also a M365 Group as well. Further, if it has the Team icon in the 'Team Status' column, then it is also a Team.
scenario 1 - Sharepoint site not listed on the Group page, then migrate as a Sharepoint migration
scenario 2 - Sharepoint site listed on the Group page, with an empty 'Team status' column, migrate as a M365 Group first . And then migrate it as a Sharepoint site as well.
scenario 3 - Sharepoint site listed on the Group page, with 'Team status' column containing the Teams icon, migrate as a Team first. And then migrate it as a Sharepoint Site.
Important - If the Sharepoint site is a Teams site (including just a M365 Group) and the 'Site Creation' configuration is creating Team sites under a different folder structure for the target than exists on the source (i.e.'Create Team Sites Under' "/Teams" instead of "Sites"), then you need to perform the Manual Mapping the Sharepoint migration _first_ before doing the Group migration. Doing the M365 Group migration first would result in the Sharepoint migration not able to find the site on the target.
Note: If the