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I entered the non-linear adjustment function gnls within a loop for so that it could test a series of start values automatically.
The point is that eventually some of these start values create a routine error gnls. 
What I need is to get, in case of error, the loop starts with the next value, ignoring the problem. 
Something like on error goto next, but there is no such syntax in R.
see
?tryCatch()– djas
Thank you so much, @djas! It’s working!
– José Ricardo
@djas put your comment as a reply, abs
– Carlos Cinelli