Welcome to this user forum.
First try restarting the BT YouView box, as that can help.
What signal level and signal quality is the box showing?
So you've got about 90% signal strength which is maybe a little low as 95% would be better.
And about 20% signal quality which is terrible.
Have you tuned into the correct transmitter/region?
Do you know how far you're from the main transmitter? If you're very close it could be that the signal is too strong and you need to do something about weakening it, as the Youview's tuner can't well handle really strong signals.
You could try adjusting the attenuator to drop the signal level slightly, to see if it improves the quality. It may not help, as I think you have a poor quality signal entering the booster.
The problem is that signal boosters do not normally improve the quality, as they introduce a small amount of noise.
It may be that you need to improve your outside aerial to get a better signal from the transmitter. A good aerial company should be able to do this properly, as they have signal level meters.
Okay thanks for your help, is there anyway to simply turn the message off in the mean time?
You may be able to get rid of it by slightly adjusting the signal level on the attenuator, or there may be a gain adjustment on the booster itself.