Thank you guys for your replying.
I made just a Data asset and I am not sure what Asset Manager is. Do I need to register my assets to it in order to access them in C++?
About selecting, I added a C++ script which has UPROPERTYs as I said, and made a blueprint from the script. In that blueprint(double click the blueprint), I can see the selection menu as below.
But I cannot select any SpawnableItem_DataAsset although I have 2 datas which I made from Miscellaneous > DataAsset > SpawnableItem_DataAsset.