When refitting the valve cover you cam chain tensioner should NOT be fitted. you will have noticed when timimg the cams that they do not appear to quite line up properly. There is a reason for that. In the middle of the inside of the valve cover is a rubber skid. When the valve cover is fitted this skid pushes down the cam chain inbetween the camshafts and ‘corrects’ this ‘error’.
Make sure the slack in the cam chain is inbetween the camshaft sprockets. This part should not be tight as you will struggle to get the valve cover to sit flat and seal.
Once the cover is bolted down refit the cam chain tensioner and slowly turn the engine by hand to transfer the slack left between the camshafts to the front of the engine then adjust the cam chain tensioner so that the end plunger is flush with the end of the body at its innermost position.