Before You Begin
Make sure that you have Removed the Add-In/SPFx from Site Collection Recycle Bin
Step 1: Login to your Office 365 admin center page with administrator account, then choose Admin
Step 2: Go to SharePoint Admin Center
Or in the new admin center
Step 3: Open App Catalog page
Click on Classic features on the left menu section:
Click on Open button under the Apps section:
Click the App Catalog: