Hello,

This is because you currently have the "Scrollable" option enabled.

Please check out this Documentation, which explains more details about Table layout and table word wrap.

The most important part says :

The “Limit Table Width” option is only shown once the “Scrollable” feature is disabled, and vice-versa. 

If you don’t see “Limit Table Width”, make sure that the “Scrollable” option is deactivated.

There is only one custom workaround if you wish to have "horizontal scroll"/scrollable option while being able to set up Custom Column Width ( in that case, word wrap still won't be possible)

If you wish to try that, here are the steps.

First, enable "scrollable" in the Display tab, then you can add this custom CSS to the table's customize / Custom CSS tab :

.wpDataTablesWrapper table.scroll.wpDataTableID-1 {  
  width: max-content !important;   
 table-layout: fixed !important;
}
.wpDataTableID-1 th.column-strap { 
    width:300px !important;
}

You will have to replace the table ID 1 with your table's ID,

and for the column name, you have to set the "origin header name" of the column, after this part : "th.column-"

For example, on my test table here,

if i wish to add the custom width in this CSS for the "firstname" column,

it would be 

.wpDataTableID-2671 th.column-firstname {   
  width:300px !important;
}

I hope that helps.

Let us know if you have any additional questions.

Thank you.