A few comments:

• I have always found that tipping the slide slightly, while lowering the cover slide at an angle allows any bubbles which may form to migrate to one corner/side, making it easier to remove them.
• The liquid solution that you are using (you did not say what it was) may be too viscous with high surface tension, resulting in trapped bubbles. Can you thin it?
• Can you let the solution sit for a few minutes before dispensing onto the slide? Maybe any trapped gas will release to the top surface if you wait?
• Are you able to warm the slide slightly? Gentle heating may degas the solution.
• Have you discussed this concern with your TEACHER? They are in the best position to help you in-person at your school, not someone on the web.

After completing the preparation of the sample, place it in a volumetric cylinder inside ultrasonic water bath, It may help prevent bubbles

My main question is why do you calculate the exact volume needed to fill the sticker cavity. It might be not enough.

Try to use a 5% or 10% extra volume to have a resulting drop in the cavity slightly higher than the sticker surface. Then place the cover edge on one site and slowly lower it till it rests on the whole sticker, allowing to flow a little surplus of sample fluid over the rim. Good luck