Error: The specified neighbor cell in [GLTENCELL] does not exist

LTE cell should be added as external cell first using ADD GEXTLTECELL.

CellIdentity (CI) used to uniquely identify an LTE cell in the PLMN. For macro base stations, EUtran CellId consists of a 20-bit eNodeBID and an 8-bit CellID (EutranCellID = eNodeBID x 256 + CellID). For micro base stations, EUtranCellID is a 28-bit eNodeBID.
This is for Huawei.

Check this for other vendors too: