The further delay triggered a furious reaction from aviation chiefs, who said the Scottish Government should put up or shut up.
Public finance minister Kate Forbes said the planned 50 per cent cut by replacing air passenger duty (APD) with an "air departure tax" (ADT) could not be implemented until continued exemption for Inverness airport was secured.
APD is currently £13 per passenger on flights from the UK of up to 2,000 miles, and £78 for longer distances.
The minority Scottish Government - which does not have support for its plans at Holyrood - had hoped to make the reduction a year ago.
Answering a written Parliamentary question from Renfrewshire South SNP MSP Tom Arthur, she said: "A solution has not yet been found that would be ready for introduction at the beginning of the next financial year.
"This, taken together with the continued uncertainty around Brexit, means that that we have to defer the introduction ADT beyond April 2020."
Ms Forbes said to do otherwise "would compromise the devolved powers and risk damage to the Highlands and Islands economy".
BACKGROUND: Scottish Government admits air tax cut could be delayedBut exasperated airline and airport chiefs said ministers must be frank about the prospects of the tax cut ever happening.
Tim Alderslade, chief executive of Airlines UK, which represents carriers, Derek Provan, chief executive of AGS Airports, which owns Glasgow and Aberdeen airports, and Edinburgh Airport chief executive Gordon Dewar said: “The Scottish Government needs to be straight with industry.
"This was a cast-iron manifesto commitment and they have now failed to implement it two years in a row.
"In the meantime, it is Scottish tourism and connectivity that is suffering, as we’ve seen with Norwegian pulling out of Edinburgh, and lost routes at both Glasgow and Aberdeen.
"The message from airlines and airports is clear – either do what you have promised and get on with it sooner rather than later or be upfront with us that it is never going to happen.”
The Scottish Liberal Democrats called for the plans to be scrapped.
Energy spokesman Liam McArthur said: “While Ian Blackford joins other party leaders in meeting environmental activist Greta Thunberg, SNP ministers are sat squarely in the pocket of airline lobbyists.
"They are proposing pumping tens of thousands of tonnes of additional emissions into the atmosphere.
“Enormous tax cuts for the aviation industry will hurt our efforts to tackle climate change and leave less money available for public services.
"It’s time the Scottish Government abandoned this anti-environmental policy.”
Scottish Greens Parliamentary co-leader and finance spokesman Patrick Harvie MSP said: “The SNP’s increasingly absurd attachment to the idea of a massive tax giveaway for the airlines is unfair to the majority of people who rarely fly, out of step with climate reality, and clearly undeliverable.
"They should stop pretending that it’s going to happen, and instead invest in the affordable public transport that people in every community in Scotland depend on.”
A Scottish Government spokesperson said: “We want to protect the existing Highlands and Islands exemption, but a solution has not been found that would be ready for introduction in April 2020.
"Air connectivity is critical for the Highlands and Islands and we are determined to deliver the best possible outcomes for the area.
“The Scottish Government will continue to work with the UK Government and the Highlands and Islands Working Group to seek a solution.
“We continue to see growth in overall passenger numbers, but continued uncertainty around Brexit and unfavourable exchange rates are having a negative impact on route development in Scotland.
"When routes are withdrawn, a variety of factors, including the UK’s EU exit, currency fluctuations and fuel costs, in addition to APD, all influence performance and decisions.
“We remain committed to reducing ADT by 50 per cent and abolishing it when resources allow.”