Putting it together: we cannot assist concerning the issue.
"With" – concerning works.
For "can’t", maybe cannot – but "can not" is the full form, which is two words. The original is a contraction. Alternatively, "cannot" is correct. So cannot as before. Putting it together: we cannot assist concerning the issue
I cannot assist concerning the matter.
"That" – it maybe. Wait, the user said to skip brands and names, but "the issue" might be okay as a noun. "the matter". Hmm
"That" – alternatives could be "it", "that", "something", "the matter". Hmm, "it" is straightforward. Maybe it? But "that" is the original, so perhaps include it as one of the options. So it. Putting it together: we cannot assist concerning the issue