[SOLVED] macOS 10.13 to 10.13.6: com.apple.osinstall error -1

I got a Macbook Air 13 (2017) recently, that had macOS High Sierra 10.13 installed on it by default. Unfortunately the new 10.13.6 update didn't work neither from App Store nor from Apple's official website (combo update). The following error message showed up in both cases:

macOS could not be installed on your computer

The operation couldn't be completed. (com.apple.osinstall error -1.)

After googling for hours, found a possible cause of error: the FileVault disk encryption prevents installing the update.

Solution:

1. Put your MacBook on charge

2. System Preferences -> Security & Privacy -> FileVault

3. "Click the lock to make changes." in the bottom left corner -> type password

4. Then click on "Turn Off FileVault..." button

5. Wait until it decrypts your disk (may take hours)

6. Install update without any error