Can't install an app in macOS High Sierra because of incorrect password
Apparently your system doesn't have an Admin account. You can create one anew. An easy way to do this is to re-run the Setup Assistant. Follow the below mentioned steps:


Start by shutting down your Mac. If you are not able to shut down normally, simply press and hold the power button to turn it off.
Now, you need to start your Mac in a special mode called Single User mode. To start your Mac in Single User mode, simply press the power button to start, and press and hold the Command + S keys.
Let your Mac complete whatever it is doing (for e.g. displaying scrolling lines of text). Once done, you'll be shown a command prompt of the form

:/ root#
Now, you'll need to manually mount the startup drive. To do so, type the following command line and press Enter:

/sbin/mount -uw /
Now, we'll delete a system file. This will make the Mac re-run the Setup Assistant. Type the following at the command line and press Enter:

rm /var/db/.AppleSetupDone
Now you need to reboot your computer normally. To do so, type the following and press Enter:

reboot
Now, let the Mac start normally. It will now start the Setup Assistant. Use it to create a new Admin account.


Once you have a new Admin account, you'll be able to perform administrative tasks and move files from the Standard account to the new admin account. You can also convert the Standard account to admin, and get rid of the freshly created admin account.