Both air pockets under the wallpaper and creases are usually the result of too short a soaking time or softening time of the paste, in the case of wallpapers where the paste must be applied to the wallpaper. How long the paste must soak can be seen on the product sheet enclosed with each roll of wallpaper. You should always treat the manufacturer's instructions as obligatory instructions. If the wallpaper does not get the soaking time it needs before it is applied, it will continue to expand on the wall, thus creating air pockets or creases.
If, after drying, an air pocket has still appeared that can no longer be "pressed out", you can cut open the affected area with a box cutter. Two small crosswise cuts are sufficient. Then apply fresh paste under the edges and carefully smooth everything out again. However, if this trick does not help with creases, you will need to completely replace the strip.