Your card may be full. If you get an error message “card cannot be synced” while trying to transfer and sync charts from your Boating app to your chartplotter, this is because there's not enough free space available on your card.
If the current charts exceed the capacity of the card, you will need to get a new larger Navionics card to use Plotter Sync.
You can buy a new card from a dealer
To know the amount of space you have used on your card please check this article: How much information can I download to my card?
If your card is full and you want to make sure you don’t have unnecessary files saved check this article: Chart Installer Error: Card is Full/Not Enough Space on Card