MSEG / NSDM_V_MSEG needs to be adapted; see SAP Note 2242679

What causes this issue?

When you add a new field to the coding block, table MSEG is initially
inconsistent because compatibility view NSDM_V_MSEG does not contain the
new coding field automatically.

System Response

Read or write accesses to table MSEG lead to runtime errors.

How to fix this error?

For fields that are <ZH>not</> intended for CDS-based analyses, create a
new extension view for NSDM_V_MSEG as described in SAP Note 2242679 to
include the field in the compatibility view. To avoid inconsistencies in
receiver systems, use the same transport request as for the coding
block.
For fields that are intended for CDS-based analyses, the system
generates an append structure automatically when you use transaction
SCFD_EUI to add your user-defined field to your Fiori app. This append
structure resolves the inconsistency. To avoid inconsistencies in
receiver systems, use the same transport request as for the coding
block.

Error message extract from SAP system. Copyright SAP SE.