Sometimes the pack is installed but your inventory or buttons still look like default Minecraft. These checks cover the usual causes.
Check the pack list
1. Open Options, then Resource Packs.
2. Make sure AE2 GUI Resource Pack is in the Selected list on the right. Being in the Available list on the left is not enough.
3. Look for any pack above it that also changes menus or buttons. Higher packs override lower ones, so move AE2 GUI up past them.
4. Click Done.
5. If nothing changed, press F3 + T in game to force a full resource reload.
Other things to rule out
- Server resource packs: Some multiplayer servers send their own pack, which can load on top of yours. If the menus look right in singleplayer but not on one server, that server's pack is likely covering the interface.
- Partial styling: Check the screens the pack is known to change, such as the inventory, crafting table, smoker, loom, brewing stand, advancements, buttons and text boxes. If only some of them look vanilla, a higher pack is probably replacing just those parts.
- Wrong expectations: The pack never changes blocks, items or mobs. A world that looks unchanged is normal. Only the interface should be different.
After each change, reopen your inventory with E to confirm whether the tech design now appears.