Since this book is a fairly comprehensive text in harmony, you should keep what you've learnt from this book in mind - not only for these counterpoint pastiches you're writing, but for everything you write from now. The reason why I'm mentioning this is: 18th-century counterpoint (which is what the Goetschius book deals with) and all of the counterpoint-employing music that comes after, is completely interwoven with harmony. All of the harmonic devices/features you have learnt so far, you should find readily in these inventions. And so with this in mind, you can hopefully see the two major problems:
There are no strong cadences (V-I in root position etc.) anywhere in the piece. This is the musical equivalent of writing a paragraph of text with no punctuation whatsoever.
Some of what you write is either harmonically ambiguous, or does not follow common harmony rules. Examples:
- In bar 6, what are the first two crotchets supposed to be? Is this V-IV? This is a forbidden progression. Is this I-IV? Then why is the root of I missing? Contrast this with the last two crochets of bar 7, which clearly spells out a C major chord and is well-written.
- What are the last two crochets of bar 12 trying to spell out? Is this V? vii°? i?
- What is bar 16? You start off with a G chord (fine), introduces the C# in the upper voice which strongly suggests a chord that is the dominant seventh in third inversion of D minor (also fine), but then this dominant seventh resolves to a B natural chord (?)
Point 1 can be easily fixed. Regarding point 2: if you look at Bach's 15 Inventions, you will find that 14 of them have semiquaver prevailing rhythms, and the remainder uses broken chords extensively. This is completely deliberate in 2-part writing. Writing in semiquavers gives you more notes to work with, and one advantage of that is it allows you to trace out chords easily thereby making your harmony unambiguous. I would recommend a similar approach here.
The other problem here is form. The main material in a 2-part invention is a section of invertible counterpoint, which is then repeated but often inverted (in the sense of two voices exchanging the material they play) and/or transposed, often called the theme. You have indeed written this. But you also need material between these sections, called episodes. These have multiple functions: they serve as modulatory material, they provide a break from the theme, they introduce devices not often found in the theme such as sequences, they allow motifs found in the theme to be presented in a new context (e.g. harmonised differently), they facilitate strong cadences mentioned above, and so on. You need to write these episodes in for your invention to adhere to the form.