I’m finishing up wiring a sub panel in an outbuilding. There is an underground run of PVC conduit with THWN, which comes up to an LB on the exterior wall. On the inside I used flexible conduit so I could have a tight bend that comes up to the breaker box. Is this code? I can’t find anything in the NEC about this specific instance. I’ve attached a photo for reference.
Liquid Flex May be Okay
NEC 2017 356.1 Liquidtight Flexible Nonmetallic Conduit -- Basically says you're good but I cannot tell the size of the flex. It looks to be 1in. diameter, which is fine if that is the case. -- You may need to secure every 3ft or less to be in accordance w/ 356.30 for securing runs longer than 6ft if that were to happen.
However, you have other issues to attend to...
Disconnect Means Missing
NEC 2017 Part II 225 Buildings or Other Structures Supplied by a Feeder(s) or Branch Circuit(s) require a Means to disconnect all ungrounded conductors per 225.31 -- Basically anytime you have a separate structure fed by a branch circuit or feeder, you need a disconnect means AT THE STRUCTURE to disconnect all ungrounded conductors (and no more than 6). This is often done by installing a main-breaker type panel as opposed to a main-lug which you have used as shown in your pictures. The NEC does not define structure and as such a piece of wood driven into the ground with a sub-panel on it could qualify as structure to the authorities having jurisdiction.
Back-feeding breaker as a "Disconnecting Means"
Per NEC 2017 408.36 (D) Back-fed breakers used as a Disconnecting Means MUST be bolted or secured in place as to not be pulled out. -- Basically this means you cannot just stick in a 60A breaker in you current panel and use it as a disconnecting means because it is not bolted in place. You could use a hold-down kit as mentioned in your comment, but I think you'd do better with a bigger main-breaker panel.
You still need a Disconnecting Means in the outhouse in addition to the rule of 6
Concerning your comment "...Apply 225.33 (A)" -- To be clear: Every structure requires at least (1) disconnecting means but no more than 6 to disconnect all ungrounded conductors. 225.31 has (4) exceptions to the location of the disconnecting means, but none are related to the rule of 6.
NEC 2017 352.46 Bushings for PVC based Connectors & NEC 2017 300.4(G) Insulated Fittings -- If the feeder conductors are 6 AWG or larger you will need a bushing for the PVC connector entering the bottom of the panel box.
Given the large size of the feeder you are bringing in here, that subpanel is very, very, very small. This is exactly what we warn people about: a situation where you run out of usable breaker spaces and now have to go to some serious expense or inconvenience.
If you're thinking "well, well, I can just use double-stuff breakers and cram 16 breakers in this 8-space panel" -- not anymore. Double-stuff breakers have gone the way of the dodo bird, as so many circuits do or will soon need AFCI, GFCI or both. Those breakers are not available in double-stuff.
The disconnect switch requirement will already force you to take this back to the store and get one with a main breaker. Now, put in just a few dollars more and get a 16-space or 24-space. Some of them even include "bonus breakers" as part of the combo.