Problem that causes progress to come to a complete halt

A term used in the software industry is showstopper:

showstopper: n.

A hardware or (especially) software bug that makes an implementation effectively unusable; one that absolutely has to be fixed before development can go on. Opposite in connotation from its original theatrical use, which refers to something stunningly good.

So you could say, "This is a showstopper, so it needs to be resolved before we can make any progress."

Drop the word "issue" and say "This is a roadblock." which means:

something that stops progress

[Merriam-Webster]

This is a blocking issue:

the action or process of obstructing movement, progress, or activity, in particular.
^{via Google}

We call these a Blocker

blocker: n.

A task or problem that halts progress on a project.

"This is a critical issue."

According to ODO

3.2 Having a decisive or crucial importance in the success, failure, or existence of something:

"temperature is a critical factor in successful fruit storage"
"getting banks lending again was critical to any recovery"
"time-critical tasks"

For example:

BSOD or Blue Screen of Death is a serious problem seen on computers running Windows OS.

How To Geek's article "Everything You Need To Know About the Blue Screen of Death" (emphasis mine)

BSODs appear when Microsoft Windows encounters a critical error it can’t recover from, requiring a reboot and possibly resulting in lost work.

...

A blue screen occurs when Windows encounters a “STOP Error.” This critical failure causes Windows to crash and stop working. The only thing Windows can do is stop the computer and restart it.

Another example in the field of project management.

A critical task is defined, by Dictionary of Construction, as

A task or activity that lies on the critical path of the schedule, which if delayed will delay the completion of the project.

_{^{synonyms crucial, vital, essential, all-important, paramount, pre-eminent, fundamental, key, pivotal, decisive, climacteric, momentous}}