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.