Cannot install iPadOS on iPad Pro 10.5-inch
Cannot install iPadOS 15.01on iPad Pro 10.5-inch
iPad Pro 10.5-inch, Wi-Fi, Cellular
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There are known issues with update of some iPad models (such as the iPad Pro 10.5) to iPadOS 15.0.1/15.0.2/15.1. Do you have a WiFi Only, or WiFi+Cellular model of iPad?
For affected WiFi+Cellular models of iPad, many are reporting that removal of the SIM tray from the iPad will allow the verification and installation processes to successfully compete. Other reports suggest that installing a SIM card (whether active or not) also permits the update process to complete.
You may need to restart the update process, but removing the SIM tray from the iPad is simple and costs nothing to try.
Remove or switch the SIM card in your iPhone - Apple Support
If you have a WiFi Only model, the current workaround is to remove the iPad Passcode - and then re-try the installation process. This has also been observed to resolve issues with installing/verifying the update on WiFi+Cellular models.
Be aware that removing the iPad Passcode has implications for any payment methods registered for ApplePay from iPad - and your Keychain (that includes your saved passwords). Assuming that you have other devices (such as an iPhone), your Keychain will remain preserved in iCloud - and can be easily re-synchronised with your iPad via iCloud. Payment cards registered for ApplePay will need to be manually re-added after the update.