I can’t insert a page of libreoffice writer (.odt) with text and an image (odt) into an impress (.odp) slide presentation
If it contains an image, it cannot be converted to a text shape. You can insert it as OLE. Insert > Object > OLE > Create From file > Search. You should link to the .odt file, because then changes are easier and the text document may contain OLE (e.g. formulas, chart) itself.
But unless the text document itself is topic of the presentation I would not try to insert it. Text documents are usually in portrait and use font size between 10pt and 12 pt, whereas presentations are in landscape and font size should be more than 20pt. That does not fit well.