Don't worry, it only means the trailer's support wheel (or support stand) is down.
You may have noticed them in real life: these are the supports that keep trailers level when they are not attached to a vehicle. The game calls them "outriggers".

Like this: https://www.trailers-atv.com/wp-content/uploads/2016/08/Kole%C4%8Dko-pro-p%C5%99%C3%ADv%C4%9Bs-za-%C4%8Dty%C5%99kolky-.jpg

You just need to retract the support by pressing [Left-Shift]-[Q] for a few seconds, You will hear a small motor noise, then get a message saying outriggers are fully retracted.

If you want to drop the trailer, you will have to deploy the outriggers by pressing [Left-Shift]-[E], and then you can detach the trailer from the truck.

The [Left-Shift] key is called "trailer modifier", it is used for trailer functions.

Originally posted by Chacal:

Don't worry, it only means the trailer's support wheel (or support stand) is down.
You may have noticed them in real life: these are the supports that keep trailers level when they are not attached to a vehicle. The game calls them "outriggers".

Like this: https://www.trailers-atv.com/wp-content/uploads/2016/08/Kole%C4%8Dko-pro-p%C5%99%C3%ADv%C4%9Bs-za-%C4%8Dty%C5%99kolky-.jpg

You just need to retract the support by pressing [Left-Shift]-[Q] for a few seconds, You will hear a small motor noise, then get a message saying outriggers are fully retracted.

If you want to drop the trailer, you will have to deploy the outriggers by pressing [Left-Shift]-[E], and then you can detach the trailer from the truck.

The [Left-Shift] key is called "trailer modifier", it is used for trailer functions.

Thanks for the reply. The problem is when I get this message on my screen the truck won't move. It only moves when I detach the trailer.

Yes, of course. You can't drive around with the support legs down. You must first retract them.

Originally posted by Chacal:

Yes, of course. You can't drive around with the support legs down. You must first retract them.

Thanks. I didn't know they were down.