Formulas are kinda close to programming, so you can't just throw things together in whatever order and hope for it to work. You need to pay attention to what each function you call needs to take (and how much, and in what order) and what each function returns as a result. Looking at the help page on Formula Operators and Functions is a good habit to get into. There's also a quick help reference for functions that you see on the right-hand side of the window where you write your formulas.
OR() is a Boolean function. It takes at least one Boolean (true/false) input, or expressions that return Booleans (e.g.
Name = "DerekF", you get a true/false from comparisons). So the following is incorrect
DATEVALUE(Activity.Activity_Date_Time__c) < Account.Account_Close_Date__c,
0 are integers, not boolean true/false (and also because those are the outputs that you want from your formula, and your
OR() isn't the last thing (not a "top-level" thing) that your formula is doing)
IF() takes 3 parameters (all 3 are required, and they must be in this order):
- A boolean expression
- The value to return if the expression is true (the "then" part)
- The value to return if the expression is false (the "else" part)
Like I said in the question I linked to in the comments, writing out what you want to do in plain language can be helpful and the resulting formula should read very similar to the plain language description.
It takes a bit of knowledge and practice to know how to draft a formula in plain language. One thing that's good to keep in mind is that your formula must only return a single result, and it must match the type that you told Salesforce your formula will return.
So the following would be incorrect because you're trying to return more than one result:
If the account close date is blank, then return 0. Otherwise return 1. If the activity date is greater than the close date, then return 0. Otherwise return 1.
In your case, the plain language description would be
If the account close date is blank or the activity date is greater than the close date, then return 0. Otherwise, return 1
turning that into an actual formula
/* Also notice that for every open paren you have, it needs to have a */
/* matching close paren. */
/* Parenthesis are mainly used to tell you where a function starts and ends */
DATEVALUE(Activity.Activity_Date_Time__c) > Account.Account_Close_Date__c
I hope it's clear how this formula is pretty similar to how we described it in plain language.