What do you mean by this? Did you mean that after you run the code you quoted, if you run source again, you get the previous source? If that is the case, it's because you haven't assigned the results. Try with source <- source %>$ mutate(...), where ... is the rest of the code and afterwards source should get updated.
I can't help you with this.
It seems to work. Almost certainly you just copy-pasted my code in the console, and hence R has no idea what is comment object, and hence considers it as the function defined in base package. That's not what I meant.
What I meant was to use it inside mutate like the previous if_else. Add a few rows with the comment element being xxxxx or zzzzz and then if you use ... %>% mutate(Blank = if_else(((comment %in% list_of_no_comment_possibilities)|(str_detect(comment, "(.)\\1{4,}"))), 1, 0)), it is supposed to work and it do work on my device.
(Note that I change 5 to 4 here, as using 5 will search for 1+5=6 identical characters. If you want to check for repetitions of some specific characters, i.e. you know for certain it'll be either x or z you can replace . inside the parentheses)
If you can't do it, please provide a reproducible example showing your attempts at the problem and the errors you are facing.