You've misunderstood a bit, which is understandable because the naming of the things is rather less than ideal for people who aren't already familiar with PA in Fallout.
There are 3 things specifically related to PA in Fallout 76 (and, directly or indirectly, other Fallout games):
1) PA workstation. This is the thing you see in your workshop build menu, the thing that's locked. It's not the PA itself. It's the workstation used to make, repair or modify PA. It's the same sort of thing as the armor, weapon, tinker and chemistry workbenches. The reason I say the naming is rather less than ideal is because the PA workstation is in the shape of a frame and a suit of PA visually fits inside it so in that sense it is a frame. But it's not called a frame. It's a PA workstation or workbench. It's obviously not a bench, but the othe workstations are benches so the word is sometimes used for all workstations in Fallout. Not great naming, but that's how it is.
If you look at the 3rd of the photos I just posted of my old camp, you can see my workstation area with a PA workstation in the left foreground. Mine has a fancy paint job because I liked it that way. There are a couple of designs you can choose from.
You're right in thinking that you need a plan for it. You'll get one by completing the quest to get the Excavator PA. Until then (and after then, if you like), you can use any of the fairly numerous PA workstations scattered throughout the gameworld.
Unsurprisingly, the game doesn't explain how you use a PA workstation. You have to get into your PA, walk to close to the PA workstation, get out of your PA, look at the PA workstation and select the "craft/modify" option (or maybe it's "make/modify", something alone those lines).
2) PA frame. This is part of the PA itself. The first picture I posted above, the small one not in spoiler tags, shows a PA frame. From FO4, IIRC, but it shows the general idea. It's the thing you get into, the powered exoskeleton, the things that make PA work.
3) PA pieces. These are the armour pieces that you attach to the PA frame and which provide the armour itself.
Since PA is essentially a vehicle, your personal tank, I'll use a car as an analogy:
PA frame --> car chassis
PA pieces --> car body panels
PA workstation --> vehicle lift in a garage, in that it positions and holds the vehicle for you to work on it.
Sorry I don't think I did misunderstand as I was referring to the PA workstation which I have yet to get the plans for, talking of plans I learnt yesterday that I needed to click on a plan in my inventory before I can actually make it. lol.
I have an armour but as yet have not found I needed it though I did need to kill a honey beast for a quest which proved tricky.
So far I can see little difference between this and Fallout 4, lot of wandering about trying to find stuff doing quests which can prove tricky as the waypoint is either very general or not there at all as I found with the Fireman quest yesterday. That is not to say I am not enjoying it but I do find the whole crafting thing a bit tedious.
I am going to try and make a neat cabin with all my stuff inside today.