Mobile game updates sometimes go wrong. The standard troubleshooting advice is "Delete and reinstall." Without a cloud link, that advice is a death sentence for your save file.
Some outdated guides mention a “Save Data Link” as a long string of characters or a file ( ab2_save.dat ). This in current versions (v3.x and above) due to: angry birds 2 save data link
This is usually a network issue. Angry Birds 2 hates VPNs and ad-blocking DNS (like Pi-hole). Disable your VPN, switch from Wi-Fi to 5G, and try again. Mobile game updates sometimes go wrong
Log in to the Facebook, Google Play, or Apple account in the game's settings. switch from Wi-Fi to 5G