Why in lua, whith metavalue __index, t = {_index} is nil?

When, in lua, I run this code, the metavalue __index is nil

t = {__index = t}
print(t.__index)
--prints nil

but if I write it like this...

t = {}
t.__index = t
print(t.__index)
--prints table ****

... it works My question is why.

In t = {__index = t} t is whatever you assigned to t befor that line. In your case you never assigend a value to t so t is a nil value.

This is basically equivalent to

do
   local x = t
   t = {__index = x}
end

As t is nil this is equivalent to

do
   local x = nil
   t = {__index = x}
end

or

do
   t = {__index = nil}
end

or

t = {}

In the second snippet

t = {}
t.__index = t

you assign an empty table to t. So t.__index = t assigns that empty table to t.__index

See Visibility Rules

Notice that, in a declaration like local x = x, the new x being declared is not in scope yet, and so the second x refers to the outside variable.

Edit

sorry but I didn't understand : with t= {text = "hello"} t.text is not nil

t = {text = "hello"} is equivalent to

do
   local x = {}
   x.text = "hello"
   t = x
end

here you assign a string value. Not a nil value as in your first example.