Not surprising, given the age of 18, although the Graphisoft tech article on Mojave suggests that it should be possible to install:
https://helpcenter.graphisoft.com/knowledgebase/84821/
You can install 18 on another Mac that has an older OS - and then copy the 18 folder from the Applications > Graphisoft folder of that Mac to your Mojave one.
AC 28 USA and earlier • macOS Sequoia 15.4, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB
