Your requirements could not be resolved to an installable set of packages.
Problem 1
- Root composer.json requires christophwurst/nextcloud dev-stable26, found christophwurst/nextcloud[dev-master, ..., dev-beta14, 12.0.0, 13.0.0-beta2, ..., v13.0.8-RC2, 14.0.0-beta1, ..., v14.0.13, v15.0.0-beta1, ..., v15.0.14, v16.0.0-alpha1, ..., v16.0.9, v17.0.0-beta1, ..., v17.0.6, v18.0.0-beta1, ..., v18.0.5, v19.0.0-beta1, ..., v19.0.9, v20.0.0-beta1, ..., v20.0.7, v21.0.0-beta5, v21.0.0-beta6, v21.0.0-beta8, v21.0.0, v22.0.0-RC1, v22.1.1, v23.0.0-RC1, v23.0.2, v23.0.5, v24.0.0-RC1, v24.0.0-RC2, v24.0.0, v24.0.1, 26.0.0.x-dev (alias of dev-master)] but it does not match the constraint.