"This function is currently not available."

Actually, the problem occurs when I plug a device into HDMI.

So, what should I do?
Could someone help me?

Thank you, it's kind of you to respond so quickly.
- To respond to "Redbart," I changed the remote... same issue.
- To respond to "Andy31200," I had already done everything you suggested (unplugging HDMI cables, antenna, and power plug - waiting five minutes), but the problem persisted. I had also performed "Factory Reset" in the menu/options, but no change either.

It's really frustrating to be stuck on the appearance of this message.

Or a bug? Try unplugging everything, HDMI cables, antenna, and power plug, wait five minutes and plug back in.
If that doesn’t work and you have access to the menu, do a "reset" or "factory reset" or "initial setup" (see menu) but you will have to redo the entire installation and settings.

--
I call a spade a spade *
*in French in the text

Depuis que je n'ai jamais réussi à trouver le sous-titrage pour les sourds et malentendants, je ne comprends rien et je m'ennuie dans le silence toute seule. J'entends toujours la même chose : cette fonction n'est pas disponible actuellement sur le téléviseur LG 5400. Merci d'avoir de bons conseils.

It must be the devil...;)
I’ll pass and wait for the opinions of other participants

--
I call a spade a spade *
*in French in the text

My solution as of June 4, 2019:

In addition to this message, I had two other issues: automatic switching between TV channels without personal control, and automatic shutdown of my TV about a minute after it was turned on.

Having purchased my TV 8 years earlier, I concluded that all these problems were due to its age, and I therefore decided to replace it.

Hello
is a button on the remote stuck?