Can I play Plus Power or Electropower or Power Tablet on my first turn if I'm going first and am not allowed to attack?
Yes you can play those. Keep in mind there are some Pokemon (like Latios-EX with "Fast Raid" attack) that are allowed to attack on first turn.
Source: TPCi Rules Team (2021-11-11)

Relevant cards

Note: All applicable cards may not be listed below.
BW1_96
SM8_172
SWSH8_236
SWSH8_236