You need powershell module
After running below , Islands mode switched to Teams Only
Grant-CsTeamsUpgradePolicy -PolicyName UpgradeToTeams -Global
if the error complains about any SIP domain
Disable-CsOnlineSipDomain -Domain domain.com
This organization cannot be upgraded to TeamsOnly at the tenant level because there is an on-premise deployment of
Skype for Business detected in 1 or more of it sip domains, azure365pro.com. Users can be individually upgraded to
TeamsOnly using either Move-CsUser (if the user has an on-premises Skype for Business account) or
Grant-CsTeamsUpgradePolicy (if the user is homed in Skype for Business online).
   + CategoryInfo         : NotSpecified: (:) [Grant-CsTeamsUpgradePolicy], ArgumentException
   + FullyQualifiedErrorId : System.ArgumentException,Microsoft.Rtc.Management.AD.Cmdlets.CopyOcsTeamsUpgradePolicyCm
  dlet
