please help me, thank you

[yyssmm]

std::tie(mesh, rendering_params) = fitting::fit_shape_and_pose( ..........), you have fitted the pose, shape model, and expression blendshapes to landmarks in an iterative way using this function, and obtained a merged mesh, but why you merge the isomap and shape_coefficients again using the following two functions:
const Mat merged_isomap = isomap_averaging.add_and_merge(isomap);
shape_coefficients = pca_shape_merging.add_and_merge(shape_coefficients);

[Patrik Huber]

I think you are confusing between running on one image, and merging between different images.

Please adhere to better guidelines for posting issues in the future if you'd like to receive help - a clear title, description and reproducible case would be a good start. Thanks.