Please follow this simple instruction if you need to install the SPFx custom app.
Sometimes you may need to use a custom app by VirtoSoftware. In this case, the installation process differs from the classic one. Follow the steps below to install the custom app to your SharePoint site.
Step 1. Click “Download” to save the app to your device (download the sppkg file from our website).
Note: To install a custom app to your Office 365 tenant you will need SharePoint admin permissions. If you don’t have the admin permissions, please contact your SharePoint admin.
Step 2. Now go to the SharePoint Admin interface to install the custom app for your tenant.
Step 3. Find the “Apps” section and upload the downloaded file to your Apps catalog.
Step 4. Enable the installed app to add and use on all sites.
Step 5. Then Allow to use it in Teams: Teams Admin Center — Teams apps — Manage apps.
Now you can add the app to your site.