How can communism deal with worker strike?

You answered it yourself. We organize new work for these people without interrupting their employment. Perhaps we can retrain enterprises in groups, if they want to, and then have the same organization do different work. Otherwise we give them temporary positions for a while, making no change in their compensations, retrain them, and then find them another permanent solution.

where is that flag from?

This is what's categorized as a "non antagonistic" contradiction, ie one that can be resolved peacefully and in a way that satisfies everyone. A simple solution would be to downsize industries like coal, and as workers get laid off they get paid leave and access to free re-training and priority hiring in new industries. Basically you could lay off coal miners and send them to work in a solar panel factory.