What were the achievements of UN in Czechoslovakia?

Czechoslovakia was one of the founding members of the UN, joining on the 24th of October, 1945.

During the Prague Spring crisis, the US, the UK, France, Canada, Denmark and Paraguay applied to the UNSC for a special meeting. Solutions were proposed, but the presence of the USSR on the Security Council vetoed any UN action.

Immediately after the first failed resolution, Canada asked the United Nations to send a representative to Czechoslovakia to work towards the release of all incarcerated Czech leaders.

When the dissolution of the nation was imminent, the Permanent Representative informed the Secretary General that the Czech and Slovak Federal Republic (as it became known in 1990) would cease to exist and that the Czech Republic and Slovakia, would apply to membership.

They became party to all the treaties previously signed by Czechoslovakia.

Summary - Not many. As often was the case, the presence of the USSR on the Security Council blocked condemnations of atrocities behind the Iron Curtain, as the US' and Russia presence shields their allies today.