Please try the following steps to resolve the relevant issue.
1. Connection Environment
There is a possibility that the network environment may be unstable.
Please attempt to connect to a different network or at a different time to see if the same issue occurs.
2. Storage Capacity of Your Device
Please be informed that a free space for saving files is required in addition to the free space when downloading the ""Pokémon Sleep"" app.
As there may be insufficient free space for creating a save file, please attempt to free up some space and try to launch the app again.
3. Available Memory on your Device
If there are multiple apps running on your device at the same time, please try to close them all and relaunch the ""Pokémon Sleep"" app.
If it does not solve your issue, please attempt to restart your device.
4. Clear the In-Game Cache
After linking your account, select the ""MENU"" icon displayed in the upper right corner of the ""Pokémon Sleep"" title screen, confirm the message, and please try to clear the in-game cache.
*Before attempting ""Clear Cache"", please make sure that you have done account linkage by using a social media account or by your Support ID and Transfer code.
*Please be careful not to delete data by mistake.
5. Clear the Cache of the App on the Device (For Android device users)
Please clear the cache of the ""Pokémon Sleep"" app within your device by going through ""Settings"" > ""Apps"" > ""Pokémon Sleep"" > ""Storage & cache"" > ""Clear cache"".
*Please note that the operating procedure may vary depending on the device.
If you are not sure how to operate it, please consider contacting the device manufacturer.
*Also, please be careful not to delete the data.