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When a large-capacity RAM is installed, an out-of-memory error message is displayed.
Literal
Large-capacity [genitive-の] RAM [subject-が] is-installed [when-と], memory-insufficiency [object-を] indicate error-message [subject-が] is-displayed.
A bug-report or IT issue description. 搭載されている ('is installed,' 'is loaded/mounted') is the resultative passive of 搭載する ('to install, mount, equip'), used for hardware components and loaded programs. 示す ('to indicate, show') in its dictionary form as a relative-clause modifier: メモリ不足を示すエラーメッセージ = 'an error message that indicates memory shortage.' ~と here is the automatic-consequence conditional — 'when X, then Y (as a rule).' The whole sentence describes a bug ironically: you'd expect more RAM to prevent memory errors, but in this configuration it triggers one.