The Litecoin Developers are aware of a regression introduced in Litecoin Core v0.21.3. A certain number of users may see the error message "Prune mode is incompatible with -blockfilterindex." on startup.
Locate the Litecoin Data Directory. By default this is located at:
macOS: ~/Library/Application Support/Litecoin
Linux: ~/.litecoin
Windows: %APPDATA%\Litecoin
Create a file named litecoin.conf if this doesn't already exist.
Edit the litecoin.conf file with the following text:
blockfilterindex=0
peerblockfilters=0
The Litecoin Developers plan to release a new patch. Which will automatically detect if prune mode is enabled, and if so, will not attempt to enable blockfilterindex.
Although not the same error a general "fatal error" is still there with 0.21.4 as well. A summary of how it happened: I was syncing a wallet created in 2017 and not touched since then in another PC. I moved it to a new PC (windows), installed 0.24.1 and started syncing. It threw the error popup. However, syncing kept going on at the background as I was seeing it so I didn't press the OK of the error popup and kept it on till syncing finished. It took many hours. Then I pressed OK and LT-core closed. After running it again, the same error dialog kept showing up until I did the conf fix explained here.