I am assuming it's a LG Smart TV using Web OS operating system ?
If the fresh NowTV App didn't fix it or a power down by unplugging the TV from the mains for about two minutes.
Then you could try a full factory reset on the TV (i don't own an LG TV, but i believe the factory reset option is somewhere under the TV General settings).
The main problem with a factory reset, that you will have to setup the TV again from scratch.
Nope, unless an LG TV owner has some further suggestions.
You may have to contact both NowTV & LG for further help and support.
I am assuming the error message is locked on any thing when attempting to use the NowTV App and is not happening randomly all the time ?
So I own 2 LG TVs, both have been used recently (last few days) and no issues at all on either.
What @Hrymit has done is more or less what I and no doubt you have suggested. Delete reinstall the app, then if all fails set it back to factory settings.
Can you confirm that your LG isn't a Netcast and it is a WebOS version?
Selected Smart TV models
Keep in mindThe hayu Pass isn't available on any 2012 or 2013 models, plus some 2014, 2015 and 2016 models. The NOW TV app is no longer supported on LG NetCast TVs.