I'm currently doing a simulation in Ansys Fluent where I need to combine water and maleic acid in a cavity but I'm unable to find maleic acid in Fluent Database list.
You'll need to look it up. The database has some (un)common materials but isn't a complete list of everything.
Ansys customers with active commercial software licenses can access the
customer portal and submit support questions. You will need your active account number to register.