Many build systems, such as Make, rely on build scripts that are written by users to specify dependencies. As a serious dependency error in Makefiles, Missing Dependencies (MDs) can result in compiling and linking outdated artifacts in incremental builds, preventing software project updates from being applied correctly. Many studies have explored the detection of MDs. Automatically fixing those missing build dependency errors has become an apparent but challenging task. The challenges mainly result from Makefiles having complex semantics and project maintainers declaring dependencies in a variety of ways. To address these challenges, we propose a new approach to fixing MDs called MDfixer. The core idea of MDfixer is to identify the dependency declaration style in a Makefile and generate patches for the same declaration style based on declaration graphs and automatic prompt generation. Specifically, MDfixer locates dependency declarations for targets that have errors in the Makefile based on error reports, and then builds a declaration graph for each build target with errors and identifies the target’s declaration style based on a distance metric between the target and the dependencies. Based on the declaration graph and automatic prompt generation, MDfixer generates patches with the same style for the dependencies that need to be added. We evaluated the effectiveness and efficiency of MDfixer with 25 well-known projects. The evaluation results show that MDfixer can fix all of MDs. We submitted fixes for 620 individual dependency issues across 7 projects, with six of them merging our pull requests, resulting in a total of 581 errors being fixed. MDfixer consumes an average time of 1.31 min for fixing a project, with a median of 0.013s. It can assist practitioners in the effective and efficient fixing of MDs.