You can follow the steps to purchase or try a free trial subscription. Please make a reference
If you have purchased an O365 Business subscription, you may still need to be assign SPO (SharePoint Online) license for specific AAD user/ O365 user.
Use your admin account to log into O365 admin center and select a user and assign SPO license.
After clicking on "Edit", you can choose a SharePoint license to assign it to the user.
Here is a similar issue with yours, you can make a reference
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