As @Jowennl eluded to, you can't create a shape unless it is fully closed, meaning there has to be a link from every point to the next point all the way around the shape. The SurfaceLink creates it's own points at the start and the end so if you already have points at the start and end you have to do one of two things:
OPTION 1: Add the SurfaceLink from P5.x to P28.x which will generate two additional points (let's assume they are P100 and P101). Then you will add a link from P5 to P100 (this link has zero length but it is to close the side of the shape at the start of the surface link). Then you will add a link from P28 to P101 (again, this link has zero length but it is to close the side of the shape at the end of the surface link). Then you should be able to add a shape. The downfall to this method is you end up with overlaping points.
OPTION 2: As shared by @Jowennl you can change P5 and P28 to both be auxiliary points. This will then allow you to create the Surface Link in the same way but you won't have overlapping points on either end. You will then need to add a link where L29 currently is since you won't have that when P28 is changed to an auxiliary point. Then you will be able to make a closed shape.
Either option should get you to the similar end result of a closed shape, it is just a matter of whether you are okay with points on top of each other and zero-length links.
Kati Mercier, P.E. | LinkedIn | AutoCAD Civil 3D Certified Professional
Pronouns: She/Her
Co-author of "Mastering AutoCAD Civil 3D 2013"
AU2019 Speaker::: CES321590: Analyze and Revise Existing Subassembly Composer PKT Files for AutoCAD Civil 3D
AU2017 Speaker::: CI125544: Analyze and Devise in Subassembly Composer
AU2012 Speaker::: CI3001: Reverse Engineering with Subassembly Composer for AutoCAD Civil 3D
AU2011 Speaker::: CI4252: Create Subassemblies That Think Outside the Box With Subassembly Composer for AutoCAD® Civil 3D®