A common characteristic feature of plant sieve tube cells and most of mammalian erythrocytes is

Absence of nucleus

Explanation:

Seive tubes are a component of phloem and erythrocytes are RBCs.

Mitochondria are present in sieve tube cells but absent in RBCs.

Haemoglobin is absent in sieve tube cells but present in erythrocytes.

Plant cells have cell wall so sieve tube cells have cell wall but animal cells don’t have cell wall so RBCs don’t have cell wall.

Both sieve tube cells and RBCs have no nucleus.