I Normal Summon Clock Wyvern then use its effect to summon a Token, then go into Cyberse Spartoi, then use its effect to add Cynet Fusion.
I should be able to use the effect of the SiMM Tablir in my hand because there is a Level 4 Cyberse monster in my GV (Clock Wyvern), but no "Activate" prompt appears when I try to use it.

I also tried when I have both Clock Wyvern and another Level 4 Cyberse (Lady Debugger in this case) in my GV, but again I cannot activate SIMM Tablir's effect while Clock Wyvern is in GV. But when I get Clock Wyvern out of GV (through summoning it), I can now use SIMM Tablir's effect targeting Lady Debugger.

You can reveal this card in your hand, then target 1 Level 4 Cyberse monster in your GY; Special Summon this card to your zone your linked Link Monster points to, and if you do, return that target to the hand. You can only use this effect of "SIMM Tablir" once per turn.

Are you sure your Clock Spartoi was linked when you tried to use SIMM Tablir's effect?

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Okay just tried it out, got it working, thanks.
Missed the part about it needing to be linked.