Why did you add a plus-symbol before event.value? Do you know what that means?
The + sign is used to easily convert a field value to a number explicitly, which is usually only done when the value will be used later in the script as a number, as when summing the values from several fields. (Blank fields values are returned as empty stings.) In this case, you're using the value as a string, so you should not only remove the +, but you should use the valueAsString property instead of the value property, something like:
if (event.valueAsString == "Text") this.getField("Text1 Checkbox").checkThisBox(0, True);
This will ensure that it always works. Note that the following would not:
if (event.value == "012345") this.getField("Text1 Checkbox").checkThisBox(0, True);
if the user entered "012345" since the leading zero would not be included when the result is automatically cast as a number and then compared to a string. It would work when using the valueAsString property.
Also, replace "True" with "true" (without the quotes).
I have to correct this, since a lot of what I wrote isn't relevant in a validation script, since event.value is always a string and there is no event.valueAsString property. So a correct validation script would be:
if (event.value == "Text") this.getField("Text1 Checkbox").checkThisBox(0, true);
Thank you for teaching me what the "+" does. It is very important to me to not only know what I have done wrong but why it is wrong. I appreciate it very much!