Root composer.json requires rainlab/pages-plugin 1.5.1, it is satisfiable by rainlab/pages-plugin[v1.5.1] from composer repo (https://repo.packagist.org) but rainlab/pages-plugin[v1.2.9, ..., v1.5.0] from composer repo (https://gateway.octobercms.com) has higher repository priority. The packages from the higher priority repository do not match your constraint and are therefore not installable. That repository is canonical so the lower priority repo's packages are not installable. See https://getcomposer.org/repoprio for details and assistance.
The latest version in https://gateway.octobercms.com/ is still 1.5.0, which is not compatible with October v3.0.45.
If I try forcing 1.5.1 in composer.json I get: