What’s a “missing recommended” warning, do I need to fix it?

Here, missing recommended means a field from this tool's fixed profile is absent. It is not automatically a Google requirement or a Schema.org error. Read the relevant feature documentation and supply only genuine applicable data. Do not invent product identifiers, reviews or dates to improve a score. Existing empty values and absent properties may also receive different treatment under these local rules.

References: Schema.org data model; Google structured-data guidelines.