Invalid inputs

• Condition must evaluate to a single TRUE or FALSE

A single number gets coerced to a logical type.

if (56) 1
#> [1] 1
if (0.3) 1
#> [1] 1
if (0) 1

If the condition cannot evaluate to a single TRUE or FALSE, an error is (usually) produced.

if ("text") 1
#> Error in if ("text") 1: argument is not interpretable as logical
if ("true") 1 
#> 1
if (numeric()) 1
#> Error in if (numeric()) 1: argument is of length zero
if (NULL) 1
#> Error in if (NULL) 1 : argument is of length zero
if (NA) 1
#> Error in if (NA) 1: missing value where TRUE/FALSE needed

Exception is a logical vector of length greater than 1, which only generates a warning, unless you have _R_CHECK_LENGTH_1_CONDITION_ set to TRUE.
This seems to have been the default since R-4.2.0

if (c(TRUE, FALSE)) 1
#>Error in if (c(TRUE, FALSE)) 1 : the condition has length > 1