British six pence coin, unknown year, one side blank.

[Wilberforce]

It is possible to find some more information about this particular mint error?

[Pabitra]

This appears to be an intentionally made error.

Please find out its thickness as compared to a genuine coin.

When the rim is made on the blank, both sides get the rim.
This appears to have been ground on the reverse side to get a plain surface.

[Wilberforce]

Thank you for your answer.
Despite the fact that the reverse side looks almost perfectly smooth, coin compared to genuine coins indeed looks slightly thinner.

[FosseWay]

It has been ground down on purpose after (significantly after - see below) leaving the mint. What that purpose is is anyone's guess - I've seen such coins made into love tokens, advertising checks or vehicles for political slogans (with appropriate text on the plain side), tool checks (usually involving low-value obsolete coins) with a hole and stamped numbers etc., halfpennies modified for shove ha'penny... but all of the above require further modification that your piece hasn't got.

Another possibility is that it was intended to be used in some form of craft or jewellery where a smooth side was required for fixing the coin to something.

The coin saw a fair bit of circulation before being messed with, too - the colour of the metal is different on the blank side, suggesting that it saw much more use as a sixpence than it did after modification.