Hello,
Are you referring to applying inline tagging on an ASA subinterface?
If so, I do not believe inline SGT tagging is supported on logical interfaces (such as sub-interfaces).
This documentation seems to support this: https://www.cisco.com/c/en/us/td/docs/security/asa/asa93/configuration/general/asa-general-cli/aaa-trustsec.html#93713
Please look under the Layer 2 Imposition section:
Layer 2 SGT Imposition
- Supported only on physical interfaces, VLAN interfaces, port channel interfaces, and redundant interfaces.
- Not supported on logical interfaces or virtual interfaces, such as BVI.
HTH,
Ryan