Google Slides are an SVG image so it does not provide the needed context for the Co:Writer Universal Extension to work properly. As an image, Google Slides provides forward context, but does not provide backward context or cursor location.
If you are on a Mac or PC we recommend using the Co:Writer Universal Chrome Extension in a Google doc then copy the text and paste it into the Google Slide.
For Chromebooks we recommend using the Co:Writer Universal Chrome app to type out the text, then use copy/paste to enter the text into Google Slides.
***Our development team are working on a resolution so check back in the future under What's New!