You should use the buster branch. Note that the upgrade, backup and restore scripts are not working. You need to mkdir -p /etc/matrix-synapse/app-service before installing, see this PR https://github.com/YunoHost-Apps/synapse_ynh/pull/213#issuecomment-692348806 . I cannot reproduce your error about not finding ./build.sh but it looks like it goes into mautrix_whatsapp_src instead of mautrix_whatsapp
Correct go version is handled by the script. What is your architecture? I only tested on x64.
To understand the questions please have a look at the manifest file. It is better explained in the web admin.
Feel free to do a PR to improve the package!