My baby is 5 months, she is able to roll from back to front, but not from front to back. Since she is able to roll all our days are tummy time, she just refuse to lay on her back. When I put her in the cot for night sleep (I put her on her back) she always rolls to her tummy very quickly (with in 10 - 60 minutes, more often 10 than 60). In the past it was possible to roll her back when she sleeps, since yesterday she rolls to tummy even without waking up. Last night I rolls her back from tummy to her back almost every ten minutes and we both did not sleep. Can you suggest anything? She just refuses to sleep on her back.

Most babies will be rolling over by 4- 5 months. Some earlier, some later.
This is very normal as babies grow & become stronger.
Many babies then will roll & prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make her new environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Are you still wrapping/swaddling her?
If so, then swaddling, wrapping needs to be ceased.
Baby needs arms free & the space of a cot to roll well & safely.

Still always place her on back for sleep.
Ensure the mattress is firm & flat & there is nothing loose or soft in the cot.
Continue to increase tummy time when awake - this helps to build upper body strength.