An Error is shown when a user changes the Security Profile of another user: You are not allowed to assign this security profile as it contains more permissions than you have.
Vault prevents a user from assigning users to a Security Profile that includes a Permission set with permissions the user does not have.
In this case, the Security Profile of the user who wants to update the Security Profile of another user to System Administrator and does not include the System Administrator Permission Set.
Contact the System Administrator or Vault Owner to change the Security profile.
Vault Help Documentation: Related Permissions
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