See the FAQ: How to do a minimal reproducible example reprex for beginners. Unless someone has encountered the identical or closely similar problem, it's very difficult to pose an answer in the abstract. Even finding the MCMCglmm package requires some effort for those willing to read the documentation to see what arguments the function expects. (Package names are case-sensitive.)
All I can suggest is to compare the tree object to the exemplary data and confirm that the lengths of $edge and $edge.length are identical.
data(bird.families)
#> Warning in data(bird.families): data set 'bird.families' not found
str(bird.families)
#> Error in str(bird.families): object 'bird.families' not found