M Code issue: conditional cell replacement

[drburr]

I am working on setting up a report for someone that needs to show the number of days between submission and completion of a request form.  The issue that i am running into is that there are 2 completion date columns, one for completion of the overall form and the other for completion of the specific item I need to report on.  All cells in the overall form column are filled in, but only some of the cells for the specific item column are filled in.  I need to overwrite the null values in the second column with the values in the first column.

This is a sample of the data:

I have been researching to try to get this working and come up with the below coding in order to make this happen:

= Table.ReplaceValue(#"Changed Type", each [CACS 9 Completed], each if [CACS 9 Completed] = "null" then each [Date Completed] else [CACS 9 Completed], Replacer.ReplaceValue,{CACS 9 Completed})

This returns an error:

Expression.SyntaxError: Token Comma expected.

If I adjust the coding to add in a comma:

= Table.ReplaceValue(#"Changed Type", each [CACS 9 Completed], each if [CACS 9 Completed] = "null", then each [Date Completed] else [CACS 9 Completed], Replacer.ReplaceValue,{CACS 9 Completed})

I get:

Expression.SyntaxError: Token Then expected.

Can anyone help with any insight into where I'm going wrong with either coding items to achieve what I need?

[lbendlin]

= Table.ReplaceValue(#"Changed Type", each [CACS 9 Completed], each if [CACS 9 Completed] = null then [Date Completed] else [CACS 9 Completed], Replacer.ReplaceValue,{CACS 9 Completed})

lose the double quotes around null, and the second each.  Or use the COALESCE equivalent in Power Query

= Table.ReplaceValue(#"Changed Type", each [CACS 9 Completed], each [CACS 9 Completed] ?? [Date Completed], Replacer.ReplaceValue,{CACS 9 Completed})

[drburr]

I've tried both of your suggestions, and it is still returning token comma error

[lbendlin]

please post the sample data in usable form.