While adding a new cassetteI cut my new chain a little too short.The CHAIN WILL SHIFT to the large back sprocket and small front sprocket but barely with difficulty and noise.Should I add one link or 2?

No pictures... can't say.... but would it hurt to add a link or 2 and try it out? How about remeasuring the chain bypassing the rear derailleur.

Do you have a large front chainring also? If so, you need to add a lot of links or preferably get another chain. The chain should be long enough to shift into the big front and big rear at the same time.

That is very important. If your chain length won't allow big-big, there is the potential for a lot of damage if you inadvertently shift into that combination. Rest assured, if the chain won't allow it, you will try to do it some day.
If your current chain barely allows small-big, you need a MUCH longer chain.

I'd size by what the manufacturer of your rear DR says. Typically they say wrap the big big without going through the DR and then add two links. Sometimes one. For some of Shimano's road DRs they have multiple sizing methods depending on how many chainwheels and other criteria.

You should be able to find a tech doc at the mfr website that tells you.

Most 10 or 11 speed chains can't have links added, since the chains are joined with a quick link. For those chains that use a special pin for joining, the manufacturer will usually say not to put two joining pins close together.

I have to buy a new chain!Messed up the link addage!

I've been running an 11-speed chain with two quick links in it (to add 3 links) for about 2,000 miles. Removed the chain from my 2X bike when I changed chainrings and it wasn't very worn, and no sense in letting it go to waste.

add a length of the chain you cut off, between 2, quick links ..