Carry out the following steps to assign the system administrator profile to the default standard Home page:

1. Click on the Setup gear icon at the top right-hand corner of the main Home page, as shown in the following screenshot:

2. Click the Setup option, as shown in the following screenshot:

3. Type home in the Quick Find search box, as shown in the following screenshot:

4. Select the Home option.
5. Click on Set Page Assignments, as shown in the following screenshot:

6. In the resulting Select Profiles dialog, scroll through the listed profiles and check the System Administrator checkbox, as shown in the following screenshot:

7. Click on Next.
8. In the resulting Choose Lightning Page dialog, either verify or choose the System Default option (depending on your current settings), as shown in the following screenshot:

9. Click on Next.
10. Finally, in the resulting Review Assignments confirmation dialog, click on Assign, as shown in the following screenshot: