What is mitropa cup or also known as Central European cup?
This was always the highest competition, but after 1955 there was the champions league.
So my question is was mitropa cup regarded as "europa" league after 1955 or what?
I also wonder why there was no czech team in the first year of CL, since they were one of the best in mitropa cup...

Mitropa Cup was the oldest international club competion in Europe; the first to organize game between champions of different countries, back in the 1920s or so. It was only contested between a small number of central european countries (Hungary, Austria, Czechoslovakia, Italy, Yugoslavia,... and idk if any other ones). These countries would send a few teams each.

It has nothing to do with the EL or CL as it was not organized by UEFA (UEFA was founded in the 50s, decades after the Mitropa Cup was established). As soon as EL/CL/UCWC were established, the Mitropa Cup declined as countries began sending their top clubs to UEFA competitions and farmer clubs to the Mitropa Cup. Eventually the competition died.

Regarding why no czech team went to the first CL, I can't say for sure, but many FA's decided to not participate for several reasons (most of them because they thought it was a waste of time and not going to be all that interesting... they were obviously wrong and in the following years they all jumped on the wagon).

Damn that's too bad!!