It appears that you are using slashes instead of dots within the altId.
This is from the docs:
curl -X GET \
https://platform.adobe.io/data/foundation/schemaregistry/tenant/schemas/_{TENANT_ID}.schemas.533ca5da28087c44344810891b0f03d9 \
-H 'Accept: application/vnd.adobe.xed-full+json; version=1' \
-H 'Authorization: Bearer {ACCESS_TOKEN}' \
-H 'x-api-key: {API_KEY}' \
-H 'x-gw-ims-org-id: {IMS_ORG}' \
-H 'x-sandbox-name: {SANDBOX_NAME}'