He's stuck on a tough hurdle and can't get past it.
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Colloquial Beijing-flavored speech with the 儿化 (érhuà) suffix. From 坎 (kǎn, threshold/ditch) — a metaphorical 'sticking point' you have to get over.
usage
Often appears in 在坎儿上 (zài kǎnr shàng, at a critical point) or 过这个坎儿 (guò zhège kǎnr, get past this hurdle). Compare 关口 (guānkǒu, key juncture) — more formal.