I m trying to open GUI-wallet and have error "Bytecoin wallet already running".
What should i do?
Are you running any other cryptonote coin wallet at the same time?
No, only sikka gui-wallet.
Close your wallet and then open your resource monitor. See if you see wallet.exe running .. if so end the process. Then open your sikka wallet again. See if that helps.

1) I m trying, as you say, but it not helps - only "sikka.exe" running in the resource monitor.
2) Trying run sikka wallet as administrator and it change the error - Loading screen of GUI appears
(with logo) and stuck - say "Blockchain load error".
- Make sure the directory in which the wallet is located is not in a path containing non-English characters
- Make sure your Windows username does not have non-English characters. If it has, you need to specify a data directory (without non-English characters in the path) manually instead of using the default one (because the default one will be C:\Users\\AppData\Roaming\Bytecoin, so if your username has non-English characters, it's again problematic). You can do so using the --data-dir parameter.
(Also
answered on GitHub)