Because you've provided one line, I can only tell you what the error message says. First, multiple has length longer than one, and if expects values with length one. When the argument is longer than length one, if does proceeds to run normally but uses only the first element in the vector, and tries to interpret the argument as a logical (TRUE or FALSE). It is also saying that it's failed to do that.
What is multiple?
If you would like more specific help, you can post a reproducible example and describe what you want to happen.