I have found a lot of scripts trying to block youtube access but any of them is not working.

Could you please send me a working script for blocking youtube access and in general video watching?

Your solutions probably use L7 filters. These don't work anymore because YT uses SSL now. The payload is encrypted so the L7 filter cannot read the contents to make matches anymore.

Apparently the latest RC version of RouterOS has a new domain list feature (possibly only on command line) which would be the best tool for this job.