By default, during any JUNOS upgradation, new JUNOS version validiate consistency of the current active configuration with its feature library.
Hence, you may use command request system software add /...... / no-validate
to avoid such errors.
Juniper KB for your ref as well.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB12983
The no-validate turns off validation of current configuration against new Junos.
Validation, sometimes throw some lines and stops installation, sometimes it won't show anything, but it doesn't mean that current config is fully compatible with new Junos.
As an example, we had some newer version of Junos on EX. There was a VSTP+RSTP configuration. But we had to downgrade because of some issue. Older version did not support VSTP+RSTP config, only VSTP or RSTP. We did downgrade and after restart, switch didn't initialized and didn't commited this configuration and only shows that VSTP+RSTP config is incompatible, so part of our network was down.
This was just an example when validation could warn you and disble upgrade. Validate possibly works only when upgrading.