Just which preposition do you suggest?
This may be the single stupidest thing the grammar checker has ever said.
For correct usage, you may need to reword your sentence by adding a preposition directly after the marked verb or by substituting a more appropriate verb. It is incorrect to put a direct object after the marked verb.Certainly that is true for its examples (Erica listened all sides; the puppy disappeared the pancake), though 'disappear' does have a transitive use in political contexts.
But this sentence?
He hefted the package – it was light, of course – and then began unwrapping it, carefully, saving the paper and ribbon to reuse.And the underlined verb?