Following this guide:
Some of the forms for CUSTOM wallets can't be loaded.
A custom wallet is simply NOT on the list of wallets when it comes to loading the 1099-DA files.
Developers are notified about this!
As a workaround, please CHANGE TYPE of the custom wallet and then 'pretend' it's something else (Binance/Kraken/Coinbase etc.,).
The files should be loadable then.