'wouldn't be able to come' is correct and usable in written English. It is usually used to express regret about an inability to fulfill a request or attend an event. For example: "I'm so sorry, but I wouldn't be able to come to your birthday party this weekend.".
"We wouldn't be able to come in, not even to pee.
I wouldn't be able to come in if I had a wart on my nose".
I realised then and there I wouldn't be able to come to terms with an abortion.
"It would be absolutely disastrous because my family wouldn't be able to come and see me.
We won't admit you,' thousands and thousands would not be able to come to college.
"The boats would not be able to come into the creek.
Venki assumed he would not be able to come to the phone, thus revealing the prank.
When I feel like I can't trust my brain 100%, Ludwig really comes in handy. It makes me translate and proofread faster and my output more reliable.
Claudia Letizia
Head Translator and Proofreader @ organictranslations.eu