Aternos cannot add paid plugin into Aternos
there's an unpaid version of the plugin.
Posts by studiousdewsh
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MCMMO is vastly outdated and it's one of the most popular mods for minecraft that exists
https://www.spigotmc.org/resources/mcmmo.2445/ <- the latest version supports 1.12.2 and it is not available on aternos.
There's in fact very few plugins supporting 1.12 on aternos which (and i know it's free and i shouldn't complain) is super inconvenient.