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Have you checked the requirements:
Nextcloud 25
Nextcloud 26
Check your PHP version. For Nextcloud 26 you need at least PHP 8.0.
Maybe you can uninstall the problem app. Make a Backup before uninstall the app and the upgrade. You should also not wait so long for upgrades. Take a look here. Nextcloud 25, 26, 27 and 28 are EOL.
Version for upgrade is correct. 25.0.13 to 26.0.13 is correct. Then latest 27, 28, …