The MERGE statement was not available in Informix 10.00. It was added in 11.50 — see the MERGE statement in the SQL syntax manual. The new features page suggests it was added in 11.50.xC6, part of the way through the maintenance release cycle.
Note that version 10.00 and all the 11.x versions (11.10, 11.50, 11.70) are unsupported. Version 11.70 went out of support on 2020-10-01.
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