Please try the following to see if it resolves the issue.
What you can try
1. Network connection
Please make sure you have stable connection, try a different connection from the one you are having trouble with, or try connecting at a later time.
2. Storage space
In addition to the space required to download the app itself, storage space is also needed for save files. If there is insufficient space, please free up your storage and try restarting the app.
3. Device memory
If you have multiple apps open at once, try closing all open apps before restarting Pokémon Trading Card Game Pocket.
If you still cannot start up the app, please try restarting your device.
4. Clear cache
After linking your account, select the menu displayed at the top-right of the Pokémon Trading Card Game Pocket title screen, confirm the message, and clear the app’s cache.
5. Clear cache on your platform device (Android users)
Go to your device’s Settings > Apps > Pokémon Trading Card Game Pocket > Storage & cache > Clear cache.
Note: The operating procedure may vary depending on the device. If you are not sure how to operate it, please consider contacting the device manufacturer.
Please make sure you do not accidentally delete the data.