Imac OS High Sierra V 10.13.6 will not update and cannot open app store
I cannot update my Imac and I need to asap to run some graphic software.
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IF as mentioned in question - running High Sierra 10.13.6 that is because it is up-to-date. There is nothing newer in High Sierra other than what the computer is running.
If what you really mean is upGrade to a newer version of macOS - that is a different issue.
The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.
With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install. Make sure to use Only Safari to commence the download as Others may not work
Make a Time Machine Backup Before attempting. Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina and Big Sur Only support 64 bit and there is no workaround.
Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation