Hi,
I was unable to upgrade from Nextcloud 25.0.6 - 27.1.4 and resolved it thanks to the Yunohost support room:
xmpp:support@conference.yunohost.org?join
I thought it may be helpful for someone else.
Apparently there are some db migrations and intermediate upgrades to do to Nextcloud 26 prior to upgrading to Nextcloud 27
First, run this terminal command: (including full URL)
sudo yunohost app upgrade nextcloud -u GitHub - YunoHost-Apps/nextcloud_ynh at oldstable
Then once finished, the upgraded can continue from the Web Admin