Hi Alp Doğan Fidan,

Regarding the use of online synchronization in Mendix native mobile apps, there is a known limitation. You can find more details in the official documentation:  Online Sync Mode – Mendix Docs.

In this case, retrieving a generalized FileDocument directly from the mobile (client-side) is not supported due to the sync mode restrictions. Instead, you can implement a sub-microflow that retrieves and synchronizes the FileDocument object to the device. After that, you can use a retrieve action on the client side to access it.

I recommend removing the online sync mode from the involved parent and generalized entities to avoid this error.