Level 21 was pretty hard for me. It lacked the explanation that fr goes from 0 to 100 like %.
For example if you have 1fr 1fr 1fr that will give you three equally large columns that take up the rest of the space, but you can also use 0.33fr 1fr 1fr which gives you one column that's one third the size of the other two.
I'll work on improving the explanation of level 21 and fr units in general.
The gotcha is that actual value in fractional units is irrelevant as long as you defined the right number of columns.
Maybe showing more context in how the grid containers are defined?
my solution to level 21 was one 50px column, three columns taking equal share of remaining space, one 50px column.
grid-template-columns:50px 1fr 1fr 1fr 50px;