self.addParameter(QgsProcessingParameterFeatureSink(
    self.OUTPUT,
    "Extracted points",
    QgsProcessing.TypeVectorPoint
    )
)

....

   (sink, dest_id) = self.parameterAsSink(
        parameters,
        self.OUTPUT,
        context,
        source.fields(),
        source.wkbType(),
        source.sourceCrs()
    )

features = source.getFeatures()
for current, feat in enumerate(features):
    if feat.geometry().asPoint() == maximum:
        sink.addFeature(feat, QgsFeatureSink.FastInsert)

buffer = processing.run(
        "native:buffer", {
            'INPUT': sink,
            'DISTANCE': distance,
            'SEGMENTS': 5,
            'END_CAP_STYLE': 0,
            'JOIN_STYLE': 0,
            'MITER_LIMIT': 2,
            'DISSOLVE': False,
            'OUTPUT': 'memory:'
        }
    )

Incorrect parameter value for INPUT