The cause:

The error indicates that you should not have any variations in the group of factor x but you do:

t.test(y~x)

While your desire is doing this:

t.test(y,x)

Therefore, the error appears in your program.

Solution:

If you use the ~, you will have the following data:

y <- 1:10

x <- rep(letters[1:2], each = 5)

You will get this data in the case you use the ,:

y <- 1:5

x <- 6:10

I suggest you doing as the following, it will help you solve your problem.

y <- 1:10
x <- rep(1,10)
t.test(y~x) #as an alternative for t.test(y,x)