ATTinyCore has two boards with "ATtiny861".
If you select "ATiny261/461/861(a)" and load bootloader, then the message will appear:
***failed;
avrdude: WARNING: invalid value for unused bits in fuse "efuse", should be set to 1 according to datasheet
This behaviour is deprecated and will result in an error in future version
You probably want to use 0x01 instead of 0xff (double check with your datasheet first).
Difference between "0xff" and "0xfe" (previous message).
But works fine and аny program can be loaded many times!
So I use "ATiny261/461/861(a)" and the issue is resolved.