Our plans have different limits regarding the number and types of extensions, as well as the collaborator count. You can learn more about which extensions require which plan here.
If a plan shows Not eligible, why? instead of a Buy plan button in the upgrade window that means that that plan doesn't cover your current usage. Clicking on the button will tell you more about why you can't select it. There may be multiple reasons. You can always look at higher tier plans (like our Growth plans) which will support your usage or the features you're interested in.