Answer
This is usually caused by the package's declared Module/LU metadata not being set up the way the framework expects for a registered custom API: use an existing, already-registered Module constant (creating a brand-new module requires its own separate setup, not just referencing an arbitrary string) and give the LU name proper Oracle-identifier-style casing convention (e.g. ZPurchaseOrder rather than an all-caps or malformed value). After correcting those, run through the registration sequence in order: grant the package to IFSSYS, refresh the Dictionary Cache, grant the package to the relevant permission set, then refresh the Security Cache — the package's procedures/functions should then expand correctly in the Database Task method tree.