What you used looks like the package ComplexHeatmap (from the error message)? From my knowledge, ComplexHeamtap can automatically merge the heamtap dendrograms when you split by a group variable. Details see: https://jokergoo.github.io/ComplexHeatmap-reference/book/a-single-heatmap.html#split-by-categorical-variables.
You don't have to merge the dendrogram by yourself.
The problem is something wrong in your matrix (maybe too many missing values), some issues similar with you has been asked in github: