Can I update my iPhone 8 Plus to iOS 14?

You can update your iPhone to iOS 15.x

1.

You need about 7 GB device storage, see this Apple articles below:

If you need more space for an update on your iPhone, iPad, or iPod touch

https://support.apple.com/en-us/HT203097

How to check the storage on your iPhone, iPad and iPod touch

https://support.apple.com/en-gb/HT201656

Clear Other storage on your iPhone or iPad

https://support.apple.com/en-us/HT211686

What's the difference between device storage and iCloud storage?

https://support.apple.com/en-us/HT206504

2.

Check the range of the mobile network, see this Apple article: If you see No Service or Searching on your iPhone or iPad

https://support.apple.com/en-us/HT201415

3.

Check also the Wi-Fi network, see this Apple article: If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

https://support.apple.com/en-us/HT204051

4.

See also the following Apple articles:

Update your iPhone, iPad, or iPod touch

https://support.apple.com/en-in/HT204204

If your iPhone, iPad or iPod touch won’t update

https://support.apple.com/en-us/HT201435

5.

If they are On, turn Off Hotspot & VPN.