Can I say "it has a history of ... years" or "it has ... years history"?
Thank you.

Question

Answer

The best expression is:

'It has a twenty-year history.'

Note that that 'twenty-year' is used as an adjective here, and the word 'year' is in the singular. Not 'years', but 'year'.

Answer

You could also say :

It has a/an x-year history e.g. a 20-year history.

They both mean the same but this version is more concise.

Answer

You would say "It has a history of ... years"