I have a Nikon D750 and a Nikon SB-26. I like macro photography and previously used both together using slave mode on the flash. I hold the flash in my left hand and the camera in the right. But sometimes the flash doesn't trigger because my fingers blocked the sensor. I got a hot shoe to PC adapter because the D750 doesn't has a PC port. I want to use both flashes(built-in and external) to minimize shadows but when I attach the adapter and enable the built-in flash, the SB26 doesn't trigger. What can I do?
I don't think that a PC adapter on your D750's hotshoe is going to work the way you envision, because the camera only allows you to either using an external flash unit on the hotshoe or the pop-up flash, not both at the same time. Once the pop-up is popped up, the flash hotshoe is disabled (and once the camera senses a speedlight on the hotshoe, the pop-up is disabled), so your adapter doesn't actually have any signal to relay.
To avoid having your finger over the sensor, you could use the head swivel capability, so that you can rotate the body in such a way that you're gripping it without covering the sensor window. Or you could get a cheap ringlight flash, and use that instead of the pop-up flash, so you can sandwich your PC adapter between it and the hotshoe, and be able to fire both flashes.
But your best bet is probably to get radio triggers and a second flash. There's a reason so many of us use them. And this will give you a good, basic off-camera lighting set up for portraits as well as macros. They have no line-of-sight requirements (I.e., no sensor-has-to-see-the-flash-burst). And getting a second external flash will free you from having to use the pop-up flash (which is limiting you to only on-axis light from very near the lens and lower power levels, anyway).
For example, you could get a Yongnuo YN-560-TX transmitter, a Yongnuo YN-560III, and a Yongnuo RF-603II or RF-605 transceiver unit here in the US for a grand total of around $150 (obviously, there are a lot of other choices out there for this type of gear at a variety of price points and reliability reputations). The transmitter would go on your D750's hotshoe, the 560-III has a receiver built-in, and you'd put the RF-603II on the foot of your SB-26. And while you'd still have to dial in the power on your SB-26 (which you're probably already used to doing), you'd then have two off-camera lights that can be fired over radio, without the range or line-of-sight issues that optical slaving, and you won't be limited to the length of a cable.
That is kinda how it is, things in the hot shoe will block the internal flash door. Why a stupid $2000 camera thinks we don't need a PC connection is a mystery to me.
But the SB-26 (and several later flashes too) has an optical slave mode, where it is triggered in sync from the flash of any manual mode flash. SB-26 manual, page 126, Wireless Slave Flash Photography. You could use that, without the PC cord. The internal flash in manual mode should trigger it (it must be Manual flash mode, TTL mode will NOT trigger it properly).
The simplest solution would seem to be to use a Flash Handle Grip which would keep your fingers away from the flash sensor and provide a more secure way to hold the flash.