I am trying to write a validation rule to prevent users from changing Account name on the Opportunity once a value is present in a custom text field on that Opportunity.
If I use
ISCHANGED(Account.Name) I get
"Error: The ISCHANGED function cannot reference the Account.Name field."
and when I try with
ISCHANGED(Account) I get
" Error: Field Account does not exist. Check spelling."
Can you please help me with the formula?