As a power-saving feature, many (most?) TVs are now designed to switch off after a set period if there has been no operation of the set (such as changing channels or adjusting the volume) during that period.
If you're using your TV as a monitor to display the signal from a Sky Q box then changing channels (using the remote control for the Sky Q box) won't register as an 'operation' with the TV, so that (irrespective of how many times you've changed channels) the TV set will still turn itself off after the relevant time period has elapsed.
However your TV warns you that it's about to switch off, in order to give you the chance to cancel the operation. So you need to use the remote control for the TV set (and NOT the one for the Sky box) to select the relevant operation. (At a guess, you probably need to use the arrow keys to highlight 'Disable' and then the 'OK' or 'Enter' button to provide that instruction to the TV).
If you're still stuck, please provide the exact model number for your TV set (from the back of it or from the box it came in), so that we can consult the manual for it.