What's wrong? FastLED and WS2812B simulation works, physical build doesn't
Consider what would happen if your power supply is not powerful enough.  Depending on the number of WS2812B you are lighting at the same time you may be exceeding the current rating of your power source.  Using the following quote from this source:

... consider how much current your LED strip is going to pull. With every
single LED on at full brightness, each breakout board can pull about
60mA (20mA per color channel).

We can calculate for a string of 15 WS2812 set to full brightness it would take 15 x 60mA or almost 1 Amp of power.  People commonly source power supply which can handle twice the expected high current load.  So, in this case you would need a supply which can source at least 2 Amps
Also consider not using the Arduino's linear voltage regulator for large current loads.  Not only does it have current limitations but it is also not as effective as a switching power supply. Instead use a power supply of the correct voltage and capable current rating for large current loads.
Try lighting only 1 LED at a time in the strip and stick to one color at a time.  If this works it is likely your power supply is not capable of supplying the necessary current to operate your project as expected.