You have to enable custom script for your site collection.
Execute the following script in SharePoint Online Management Shell:
$spadminurl = "https://yourtenantprefix-admin.sharepoint.com/"
connect-sposervice -url $spadminurl
$spsiteurl = "https://yourtenantprefix.sharepoint.com/sites/yoursitecollectionurl" # or "https://yourtenantprefix.sharepoint.com" for your root site collection in tenant
Set-SPOsite $spsiteurl -DenyAddAndCustomizePages 0
Then you'll be able to install Virto Workflows.