Do you have 30 days temporary license? If so, it will be expired after 30 days. You may not be able to use the APIs full fledged after your temporary license is expired. Once you purchase the license (and you set/update your licensing code to refer to newer license), you will be able to use the Aspose.Cells APIs (either from nuget or Downloads section, older or newer) without any issue.
FYI, when you purchase a license for the product, you are authorized to use the license file with any new (and upcoming (official versions)) versions or hot fixes of the component for the next whole year. Moreover, your license will never expire if you continue to use your license with the product’s version/hotfix that should be released before your subscription expiry date. You may open your license file (after purchasing it) into notepad (without modifying it, lease it will not work) and check your subscription expiry date for confirmation.
Hope, this helps a bit.