Download linux-image-unsigned-5.1.0-994-generic_5.1.0-994.201904222200_amd64.deb and linux-modules-5.1.0-994-generic_5.1.0-994.201904222200_amd64.deb then install them.
Download linux-image- unsigned- 5.1.0-994- generic_ 5.1.0-994. 201904222200_ amd64.deb and linux-modules- 5.1.0-994- generic_ 5.1.0-994. 201904222200_ amd64.deb then install them.