When using a cracked server, players may not see their skins because the server doesn't authenticate with Mojang's servers. However, there are ways to enable skins.
Install SkinRestorer:
There are several plugins available that try to restore Minecraft skins on cracked servers. The most popular of these is SkinRestorer. You can easily install this plugin from our plugin library.
If you don't know how to install plugins, check out this article for instructions.
Note:
The other option is to disable the cracked setting here: https://exaroton.com/options/ (if all players have a premium Minecraft account).