If you wish to allocate quota to SharePoint Online site collection, you need to have a SharePoint Online admin account.

1. Firstly, login to SharePoint admin center by clicking on Admin -> SharePoint.

2. Select settings. From Site Collection Storage Management, select Manual then click OK.

3. From Site Collections tab, you can choose one or more site collections from the list.

4. Select Storage Quota. If the storage quota button is not enabled, select the site collection you want to set a quota on from the list below.

5. In Limit storage quota for each site collection, enter the quota in gigabytes (GB) to be allocated to the site collection.

6. Tick checkbox to send the email to the site collection administrators when the site collection reaches the storage limit.

7. Enter the percentage of the limit (1 to 100) to reach before an email alert is triggered. Then, click Save.