Checkbox UI only field set from formula result?

Did you try true/false?

IF({{Field_Name}},true,false)

It gets weirder.  I tried to have something that wasn’t dependent on my field:
if(true,true,false)
and I tried
if(1,true,false)

Both of these should return true in every case.  I get the error:
Invalid syntax for field formula: if(true,true,false). SyntaxError: Unexpected end of input
and
Invalid syntax for field formula: if(1,true,false). SyntaxError: Unexpected end of input


Likewise other things I tried:
Invalid syntax for field formula: if(1==1,true,false). SyntaxError: Unexpected end of input

I’m thinking case sensitivity is tripping you up.  IF not if. 

Having said that.  Here are a few boolean indicators that work for me: 

IF({{Amount}}>100000,true,false)

If you are looking for true, false return - you may not even need the IF function.  This works as a formula in a UI only field.
 
{{StageName}}==“Needs Analysis”

Finally - if you want to control the display of warning messages etc based on whether some field is present - you don’t need to create a second field,  just do conditional rendering of the the warning message,  or use the conditional statements in our merge syntax to show different messages.  In a template put somthing like this: 

{{#FieldName}} Thanks for giving us the data {{/FieldName}}{{^FieldName}} Hey dummy - give us the data… {{/FieldName}}

Refresher:  
# in the merge syntax means “if there is a some value, do what’s next”
/ in the merge syntax means “that’s the end of this condition block”
^ in the merge syntax means “If there is NO value, do what’s next”

Cheers.