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If you see "Software update is required to connect to your iOS device" on your iPhone, iPad or iPod touch – Apple Support (UK)
If so, you may need to update the software on your Mac in order for it to recognise your iOS device.
Click Install to download and install the software so your Mac can update or sync your iOS device.
It is just a small patch for macOS.
You won't notice that update by the way, it only lasted 3 minutes or so.
The Mac remains 11.5 and the iPhone remains iOS iOS 12.5.6.
Also, my computer is very slow
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Check the status of your network on Mac
Check the status of your network on Mac - Apple Support
Free up storage space on your Mac
Free up storage space on your Mac - Apple Support (IN)