If you have the newest version if InDesign installed (20.x), uninstall it as it will not work with OS 11. Using the Creative Cloud app, you can install InDesign 19.x, which will work.
Adobe only allows one version back to be installed, so you might want to either:
See if Adobe can supply the InDesign 19 installer in case you need to install again. (At some point, even that version will no longer activate.)
Update your system OS. However, since you specifically mentioned OS 11, if your Mac is too old to update, this won't work.
Invest in a new computer.
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