I have a form which has a single text field that the user enters. I need to validate the form that the value entered into this field exists in another SharePoint list when the form is submitted. If it doesn't exist, error message should appear which says "Please enter the valid ID". Please suggest me some options.
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you can use lookup() runtime function within validation rule to get/check entry from the another list.
but I would suggest rather to use lookup control (or even lookup list field).
that way users will no be able to select anything else but a value already existing on the other list, and you need not to take care about validations.