As explained by the <<repeat>> macro's documentation the first argument needs to be a valid CSS time unit.
NOTE: The delay that the <<repeat>> macro will use is determined at the time that it is called, using random() will not cause that delay to vary for each looping.
There are a number of ways to solve your syntax issue, two of them being:
1. Use a (temporary) variable to store a String representation of the unit you want to randomly determine.
<<set _delay to random(2,5) + 's'>>
2. Pass the argument as an expression using back-quotes.
<<repeat `random(2,5) + 's'`>>