Can someone help me thicken this surface?

I believer your tight corners are causing the problem.  Do they need to curve back in so tight?

... see attached image... your radius of curvature is < .001mm... and you have a singular condition which creates problems when shelling/thickening.   You need to find a way to remove/change that corner and increase that min curvature to ~1-4mm (depending on which side offsetting.

BTW, that Surface-Knit1 is a BUG.. for some reason you chose the same surface and it thinks it has two surfaces  (HEY SOLIDWORKS CORP... NICE WORK!!!!!???????)