I guess by using initials DCS, you are referring to Document Cloud Services and MSO365 is Microsoft Office 365 here?
How and where exactly you are using these services, is it on web or through freeware Adobe Reader DC?
Does DCS for MS O365 require a license to fill a pdf form stored under SharePoint?
Fill & Sign feature available for both Reader DC and DC web, that you can access for free. Now coming to SharePoint files, so, is it added as an additional account within Reader DC?
Following the installation of DCS and the opening of a pdf, the fields of the form are not accessible while they are well under Acrobat or through Edge or IE.
Document Cloud Services can't be installed, as its an online service. Form fields which are fillable within Acrobat but not on web, that completely depends on the type of form, how exactly its created.
Please share accurate information to assist you in better way.