From the strptime docs (which you can access using ?strptime)
`%U`
Week of the year as decimal number (00–53) using Sunday as the first day 1 of the week (and typically with the first Sunday of the year as day 1 of week 1). The US convention.
I think the issue is that it is trying to return the Sunday of week 53, which is invalid (see the reprex of your code with the full error message below)
as.Date(x = "2020-53-1",
format = "%Y-%U-%u")
#> Warning in strptime(x, format, tz = "GMT"): (0-based) yday 369 in year 2020 is
#> invalid
#> [1] NA
Created on 2021-01-05 by the reprex package (v0.3.0.9001)