Why can't I change the field type in Marketo?

The warning is genuine. 

Request_Portfolio_Review__c: Marketo type 'string', expected 'boolean' (SFDC type 'boolean')

Any value entered in Marketo via forms, API or manually, would automatically set Request_Portfolio_Review__c to true (box checked) regardless the values.

You can map Marketo string to SFDC textarea.
Boolean accepts 1 or 0 (true or false), unsuitable to map to strings (up to 255 characters).

the problem is, when on the progressive profile form, it auto sets no value to false, so the form see's it as being answered and doesn't ask the question.

Forcing the field to be displayed at all times, perhaps combined with a small JavaScript to make it required, could be an option.

The warning is noting the difference in field type between Marketo and. SFDC. They need to match. Did you make the recent field type change from Boolean to text in Marketo or in SFDC ? If you made the change in Marketo you should make the change in SFDC or vice versa.

THe change was made in Marketo, not in SFDC, so I just need to go into Salesforce and make the same change and everythign should be all set?

Why is it make me do that when it it is categorized, under field mapping, as unmapped custom fields??

Yes, you just need to make the change in SFDC. My advice is ignore the field mappings page after the initial sync, it can lead to confusion. If you want yo see if a field is mapped to SFDC then just go to field management and search for the field and you should see "CRM field map" which will tell you if the field is mapped to an SFDC field.

thank you. The easiest thing was to just change the type back to boolean and just make it required everytime.