MacBook5.1 (13inch,Alum, late2008) - Can I upgrade macOS from El Capitan 10.11.6

No Steve, this is a list of earliest hardware units that are capable of the update to Sierra.

These Mac models are compatible with macOS Sierra:

• MacBook (Late 2009 or newer)
• MacBook Pro (Mid 2010 or newer)
• MacBook Air (Late 2010 or newer)
• Mac mini (Mid 2010 or newer)
• iMac (Late 2009 or newer)
• Mac Pro (Mid 2010 or newer)

See here for more information

Hi, welcome to the Apple Users Community.

In short, no, sorry Steven.

Mac hardware requirements for Sierra are as follows:

For details about your Mac model, click the Apple icon at the top left of your screen, choose About This Mac, then choose More Info. These Mac models are compatible with macOS Sierra:

• MacBook (Late 2009 or newer)
• MacBook Pro (Mid 2010 or newer)
• MacBook Air (Late 2010 or newer)
• Mac mini (Mid 2010 or newer)
• iMac (Late 2009 or newer)
• Mac Pro (Mid 2010 or newer)

Best of luck