The compressor is stalling on startup.
Unfortunately Mistral are very cheap, in all senses.
To test and diagnose you really need to be qualified to work on electrical equipment as some testing is done live.
It may be a faulty run capacitor (common), a bad/burnt connection on one of the compressor terminals, a faulty start winding on the compressor or the compressor is seized.
A run cap is an easy fix, any of the other faults I'd suggest a new air conditioner and preferably not a Mistral