How to add a new filed in an attribute table. After adding i have a red square, instend of green.

[YanaPrados]

I do not understand what I am doing wrong. I tried the same with other tables- it is working, however, exactly with this- it doesnt work( why?

[DanPatterson]

Park your mouse over the red square. 

The only way I could get a red square was to use a duplicate field name