The best way to do this is to look at the last sector which was running.
If you scroll up above the error message, you should see something like the following:
-- 2021-10-14 16:49:10 - muse.sectors.sector - INFO
Running power for year 2045
... etc
If this is the first message you see, it means that the power sector wasn't able to expand any further. Therefore you should relax the constraints on the power sector.
Another way of doing this is to plot the capacity and see if there is a particular technology which is rapidly expanding. A lot of the time, it is this technology which is causing the problems due to too strict constraints.
Best wishes,
Alex