Cannot Enable Salesforce Multi-Factor Authentication for User Interface Logins on Standard Profiles

Assign the "Multi-Factor Authentication for User Interface Logins" permission to users using either a permission set or a custom profile.

Using a Permission Set:

1. From Setup, enter "Permission Sets" in the Quick Find box, then select Permission Sets.
2. Click New.
3. Enter a Label and API Name that clearly indicates this permission set is for MFA for User Interface Logins, then click Save.
4. Click System Permissions.
5. Click Edit.
6. Check Multi-Factor Authentication for User Interface Logins, then click Save.
7. Click Manage Assignments.
8. Click Add Assignment.
9. Select the users to whom you want to assign the permission set, then click Next.
10. Click Assign, then click Done.

References: 

• Create Permission Sets
• Manage Permission Set Assignments

Using a Custom Profile:

1. From Setup, enter "Profiles" in the Quick Find box, then select Profiles.
2. Click Clone next to the profile currently assigned to the target users.
3. Enter a Profile Name, then click Save.
4. Click Edit.
5. Under General User Permissions, check Multi-Factor Authentication for User Interface Logins, then click Save.
6. Enter "Users" in the Quick Find box, then select Users.
7. Click Edit next to the user you want to assign the new profile to.
8. In the Profile dropdown list, select the new custom profile and click Save.