I'm using OS 10.11.6 and Mail constantly crashes on opening. How do I fix this please?

The cause is usually an email message incompatible with that Mac's older version of Mail.

Press and hold a Shift key while launching the Mail app again. That will launch the Mail app with no message selected. In that manner you may be able to identify the problematic message, and delete it.

Another way to identify and delete the problematic messages is described in If you can’t send or receive email on your Mac - Apple Support, under "Test using webmail."

macOS version 10.11.6 "El Capitan" is decidedly old. You are certain to encounter similar problems as technologies continue evolve and leave older Macs behind. If you can upgrade that Mac beyond El Cap, that will ameliorate the problem until such point those continually evolving protocols eventually make things obsolete again. Refer to Update macOS on Mac - Apple Support.

Are you using a Gmail address? We are seeing a LOT of reports receently about Macs running the obsolete macOS 10.11 no longer working with Gmail, apparently due to a system requirement change Google made.

If you have Gmail, only Google support can help you figure this out.