Check your data. If it's saying 300 is to large, then you've told the program that your actual square footage is less than 300. Remember it's the "lesser" you have to enter there.
2106 is for employee business expenses most of which have been eliminated for 2018. if your trying to take H-O on your schedule C your supposes to use form 8829, if your trying to take H-O as an employee on Schedule A forget it - it's gone.
Hum, I still don't get it. It actually says, "the lesser of the square footage of your office or 300. So, I put in 275 and it still wouldn't take it. I put in 299. It still won't take it. This section needs to be more clear.
I got the same problem in the 2019 version that. What ever the number I enter, it keep complains. It's a software bug. I can send you the screen shot.
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You're only entering one number; either 300 or the actual square footage of your home office, if less than 300.
If your home office is larger than 300, you won't use the simplified method and you'll enter that number instead.
Review your entries in that section. Make sure you indicate that 100% of your business income was earned from that home office (unless you have multiple offices).
Here's more info How to Enter a Home Office Deduction and more detailed info on The Home Office Deduction.