Try using back ticks. On a US keyboard, the back tick is just to the left of the 1 key.
library(tibble)
library(dplyr)
DF <- tibble(`N(m)` = c(1,2,NA,4,NA,6))
DF
# A tibble: 6 x 1
`N(m)`
<dbl>
1 1
2 2
3 NA
4 4
5 NA
6 6
DF %>% filter(!is.na(`N(m)`))
# A tibble: 4 x 1
`N(m)`
<dbl>
1 1
2 2
3 4
4 6