Solved

cannot do 5 stop

What did you try to do?
i wanna to set the strategy to 5 stops (6 stints)

What happened instead?
i cannot click the “+” button in order to select 5 stop at the “4stop(s)”

Did the problem happen while using wi-fi, mobile data, or both?
Wi-fi

Did the bug happen in the app, on PC, or both?
Both

What is the model of your device?
Samsung Galaxy SM-A6060

What is the operating system?
Android

What is the version number of the operating system?
11

What operating system is your PC?
iPad OS

Which browser did you experience the problem in?
Safari

Can the bug be reproduced?
yea, click the followings:
1. next race
2. strategy
3. click the “+” sign until it’s maxinum, it only shows 4 stop(s)

Additional comments:

similar stuff happened to me, i was on iPad Generation 8 by Google Chrome

That isn't a bug. There are only max. 4 stops available. IGP have to develop it. I'm sorry.
Here is the link to the suggestion for the 5th stop.
Suggestion 5th stop

But the last stop would be automatically repeated. So, the 5th stop is like the 4th.
I hope it will help you.
I know, it isn't nice but it works.

Another way is to make your tyre strategy on the starting grid.
Go to league, than to more information, perhaps you have to scroll down to the circuit and there you can go to the race after the qualy is finished. You can have max. 7 to 8 minutes to make your strategy on starting grid.
But only 4 stops too. 
Good luck and success.

so i just gotta do 4 stop with not enough laps in strategy?

That's right.
For example. Race with 60 laps.
Your strategy shows only 50 of 60 with all 4 stops.
The last (4th) stop in your strategy should have 10 or 11 laps to complete the 60 laps with the next stop.
This 5th stop and all further stops (6th...) are the same as the 4th.