Button to validate required fields

You would have to write the script yourself.

I am unsure what you want to happen. If field Text1 is empty and you click the button what do you want to happen?

This is a consequence of the way Ninox is designed. Fields can be marked as "Required" but there is no way of using that information in a trigger, as far as I know. Records are created immediately with blank fields which are then filled with data (or not), rather than the fields validated for being "Required" before allowing a record to be created.

There are various way you could handle this:

Create a formula field called, say, "Validated" with script that says something like:

if field1 = null or field2 = null or field3 = null or fieldx = null then
    "No"
else
    "Yes"
end

This will create a new field "Validated" that either says "Yes" (required fields are all entered) or "No" (some required fields are blank).  Obviously, change the script to suit your own validation criteria - doesn't have to be just checking for a null value.

Then add a View called "Unvalidated Records" to the main table filtered by Validated="No" so that you only see records with a problem.  

Or, if you use tabs on your record you could add a "Trigger after hide" to show an alert or keep the record open:

if Validated = "No" then alert("Required Fields Missing") end;

or

if Validated = "No" then openRecord(this) end;

Not ideal solutions but might give you ideas!