By default, for personal sites and self-service created sites in SharePoint Online, the option of Custom Script is disabled. To have the Solution option under Web Designer Galleries available in your SharePoint Online, follow the steps below:
Note: If you don't see the Admin tile, you don't have Office 365 administrator permissions in your organization.
- Log in as a global admin or SharePoint admin;
- Select the app launcher icon in the upper-left corner and choose Admin to open the Office 365 Administrator center;
- On the left pane, choose Admin centers and then SharePoint;
- Select Settings;
- Under Custom Script choose to Allow users to run custom script on personal site and Allow users to run custom script on self-service created sites;
- Click OK.
Info: It can take up to 24 hours for the change to take effect.