As with many abstracts, an indefinite article (and a plural ending) can be used only when the sense is either "a particular instance" or "a particular type" of the item.
There are two different senses of countable "access", neither of them very common (the iWeb corpus has about 3000 instances of "an access" followed by something which isn't a noun. It has about 20000 of "an access NOUN".)
The examples below come from the iNow corpus.
- A particular instance of something or somebody accessing whatever it is:
"... right-clicking on the file and accessing Properties is considered by the system as an access to the file"
" ... by synchronizing their execution to coordinate memory accesses to both device and thread group memory."
- A distinct method or pathway for accessing the thing.
" The left fork is an access to White Oak Drive, a private road. "
"... [the road] standard drops to single carriageway with poor horizontal and vertical alignment and many individual accesses to properties, farm fields and woodlands"
I must admit that there are not many instances there which are suitable: most cases of "an access to" there are either "an access to information request" or cases where I would not use "an". There are somewhat more appropriate cases of "accesses to".