The phrase 'unable to specify' is correct and usable in written English.
You can use it when you are unable to provide a definite answer to a question or provide additional information. For example, "When asked what caused the power outage, the spokesperson was unable to specify."
"Jay," his mother said in a warning tone, unable to specify a crime.
One thing all three main parties support is a bill of rights, but all seem unable to specify its contents.
The former chancellor said Holyrood would get substantial powers over the tax, but was left unable to specify them.
Officer Wendy Reyes, an LAPD spokesperson, told The Independent that she was unable to specify the nature of the threat.
The I.R.S. was unable to specify yesterday how much higher corporate profits were in reports to shareholders than they were on the companies' tax returns.
Due to the nature of the debt market, the group is unable to specify whose debt it purchases, taking on the amounts before it discovers individuals' identities.
In this paper we propose methods of output analysis for terminating simulations when the researcher is unable to fully specify the initial state of the simulation using observations of the real system.
The validity of interpreting experimental φ-values as the fraction of native contacts made by a residue in the TSE was explored, revealing that this definition is unable to uniquely specify a TSE.
Without the ability to defer the launch to a specified time, developers are unable to set embargo times and pre-brief press.
Many respondents believed that contemporary cutting was introduced by 'outsiders' or 'foreigners', although they were often unable or unwilling to specify or to articulate this belief more fully.
The I.R.S. was unable to state yesterday what policy it has on responding to letters asking it to specify the law that makes people liable for income taxes.
