is there a difference between "lack" vs "don't have" [closed]

In order to lack something you have to have a need for it which is unmet. It is possible to have a some of it but still have a lack of it if the amount you have is insufficient. For instance it us possible to say "He couldn't join the family in Australia because of a lack of money." This does not imply that he does not have any money, just that he does not have enough for a long haul airline ticket.

However it is impossible to lack something for which you do not have a need. There was a feminist joke at one time that said "A woman without a man is like a fish without a bicycle." The point being that, although no fish has a bicycle, none of them lack a bicycle because they have no need of one.

Don't have on the other hand means that you do not have access to or control over something. If you don't have a bicycle that means that you do not possess, or otherwise have access to a bicycle. If you are someone who cannot get to work without a bicycle and you do not have one then you lack a bicycle but if you are a fish you might not have a bicycle but you do not lack one.

In terms of your question the person who "lacks friends who will understand that experience" has a need to discuss the experience with someone who will understand but either has no friends with that capability or has too few of them. If the experience was very traumatic the person might need to discuss it multiple times and the small number of understanding friends would soon be unable to support that need.

Someone who "Doesn't have friends who will understand the experience", however, has no friends who will understand but does not, necessarily, have the need to discuss it with someone who will understand. The experience might have been traumatic and they might lack the sympathetic friends who can listen with understanding but, equally, the experience might have been uplifting or transcendent and have left the person feeling happy and liberated but without the need to communicate that feeling. In that case they may well not have friends who will understand the experience but can't be said to lack them.