I don't have diagnosed autism, but i do have severe diagnosed OCD and PTSD that has stopped me living a "normal" life. Meant having to drop out of school, no college/uni, never been able to work. Spent my teen and early adult years being almost completely housebound so no going out meeting people, all my friendships were online, and as such, so were my "relationships". 2 long distance relationships during which i had the full emotional side of the relationship, but never physical as i was too unwell to travel so never met them in person even though it was the eventual goal.18-21 with partner 1 (thankfully never met as he threatened untold physical sexual abuse while emotionally and financially and sexually abusing me via all other forms of contact) and 22-25 with partner 2, who was also emotionally and finanmcially abusive, and cheated multiple times, then strung me along until i was 28 with the possibility of getting back together (he originally left me). Both from UK but down south, i'm north.
I then met my current partner, who was only a 45 minute journey away, but during lockdown. As such we dated online only for 5 months til restrictions lifted, during which i had some improvement in my OCD, and we met in person. We now see eachother weekly, and it is incredible.
So yes, i would certainly think a lot of people with mental health conditions are negatively affected in the relationship ability department. It doesn't necessarily mean they cannot or won't have any kind of relationship at all, but it may mean they have an unconventional form of relationship, don't have relationships until later in life, etc.
Some people may genuinely just not be interested in romantic/sexual/otherwise none-platonic relationships, so lack of relationship history doesn't have to mean its not possible/difficult, but may simply be choice.