I don’t normally work on LG phones but I did have an LG G6 come in the shop recently and I had exactly this problem following a microsoldering repair. I even used the suggestions in this Q&A so I’m happy to contribute back to LG users and experts :>).

Here are my results (which may vary as every phone and situation is unique).

• I found tons of pocket lint inside the charge port. This is known to cause this error message as well but in my case, cleaning it out it didn’t solve the problem.
• I cleaned the port with 99% IPA. I wouldn’t recommend WD40 as the OP tried because while it’s great at removing moisture, it also leaves an oily residue. A bag of rice does nothing…compressed air would have been faster and more effective but in my case, there was no moisture inside the port, contrary to what the phone thinks.
• I tried activating USB Debugging mode in order to disable connection to the LG System Server —> didn’t help
• I followed @povertyd suggestion. This worked partially, allowing me to charge the device but when I rebooted the device, it was back to the Moisture message again.
• Because I had worked on the phone, I did a thorough visual inspection under high magnification of the Charge Port Flex and the inside of the charge port itself. No damage to the pins, no (more) visible debris, no dislodged components next to the connectors.
• In the end, on a hunch, I tried a different USB-C cable and voila…that was it!

My only other option left before figuring out that it was the USB cable was to replace the Charge Port Flex (iFixit doesn’t seem to carry it so here is an alternative). They are inexpensive and relatively easy to replace.