fn formula required for Nintex form

you are setting the enabled property of the field there?  did you want validation?

Maybe Do I need to use the rules section?

I need the column to only have "TBC" or a date. No other entry should be allowed.

Use validations rules instead it.

yes you need to add a validation rule.  Something like this should do it

and(not(isDate({Self})),not(equals({Self},"TBC")))

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I can't get it to work?

When I type anything in the column it accepts it. 

I noticed that I put . instead or , in the above. Amending it to , still did the same.

I think your validation rule is close to correct.  I have noticed that from the outer layer it should always seem like only one statement, i.e.   and(or(equals(a,b),"abc"),"def") 

In your example you are excluding the last bit not(equals... with a bracket to many behind isdate({Self})))

and(not(isdate({self})),not(equals({self},"TBC")))

So maybe just a bracket to many.  Let me know if this worked.

It worked - Many thanks All.