Try supported model:
https://github.com/pocopico/rp-ext/blob/main/hpsa/rpext-index.json
And read the error messages during the build.
rp-ext working, if you use supported model and dsm version.
Hello,
./rploader.sh ext ds918p-7.1.1-42962 auto
or
./rploader.sh ext ds918p-7.1.1-42962 https://github.com/pocopico/rp-ext/blob/main/vmxnet3/rpext-index.json
from pocopico/rp-ext repo (https://github.com/pocopico/rp-ext)