I find on many occassions that I can't purchase a worker even though I have enough lodgings and silver. It simply comes up with an error message that says "Cannot purchase". What are the reasons as to why I'm getting this message?

Cheers in advance.

Which cities have you tried purchasing the workers in?

if you talk about Workers market (where you purchase workers from other players), it's because he's already been sold, someone clicked faster than you ( often happens with artisan workers...)