Setup btcpayserver-docker with the opt-add-ltcmweb
fragment.
Install the plugin (or restart BTCPayServer if already installed).
Add MWEB View Keys to Litecoin MWEB Wallet settings in BTCPayServer admin.
They can be retrieved using Electrum MWEB (release-9 and later).
It is permitted to run the LTC node in pruning mode.
For the MWEB node, did you install the mweb fragment? And check that the mweb docker container is running?