I found here the reason the update is not available
Due to a regression in the PDF viewer we had to pull NC 29.0.9 from the download server. Updates to this version are not proposed anymore (but to 30 instead). We will follow up with the regular 29.0.10 next week, along other maintenance releases. RC1s can be expected this Thursday.
sudo yunohost app install https://github.com/YunoHost-Apps/nextcloud_ynh/tree/29.0.8
or if upgrading from a version below 29.0.8 sudo yunohost app upgrade nextcloud -u https://github.com/YunoHost-Apps/nextcloud_ynh/tree/29.0.8
DANGER! This app requires 512M RAM to install/upgrade but only 484.3M is availab
le right now. Even if this app could run, its installation/upgrade process requi
res a large amount of RAM so your server may freeze and fail miserably. If you a
re willing to take that risk anyway, type 'Yes, I understand'
I understand, indeed ! But don’t know how to deal with it : I can’t add more RAM or ?