"I could" is American. "I couldn't" is British.
As AmEn is now more widespread, the former is widespread, but as a native speaker of BrEn I absolutely detest it and never ever use it.
reply
ISTM that you're saying "this is a common mistake but some people don't make it."
Is that fair?
If so: no argument, but the error is now more common than the correct form, in my experience.
A BrEn example: a mistake has propagated and is now common...
"could have" [is shortened to] "could've" [is misheard as] "could of"... and the mistake gets repeated by people who don't know any better.
"I could" is American. "I couldn't" is British.
As AmEn is now more widespread, the former is widespread, but as a native speaker of BrEn I absolutely detest it and never ever use it.