To allow publishing solutions to a site, Solution Studio enables custom scripts on the site collection. This is needed because otherwise some assets (mainly scripts and web parts) required for the solution to run can't be deployed to the site.
If you need to enable custom scripts manually for a certain site, you can use the following Microsoft PowerShell commands (using the SharePoint Online Management Shell):
Connect-SPOService -Url "https://<SPO tenant name>-admin.sharepoint.com"
Set-SPOsite "https://<SPO tenant name>.sharepoint.com/sites/<Site name>" -DenyAddAndCustomizePages 0
Hint: To run this command you have to be SharePoint Admin (Site Owner is not enough)
This will allow adding custom scripts to the given site.
For more information also see the following MS article:
Allow or prevent custom script