There is absolutely no need or requirement to amend you already filed and accepted 2020 tax return.
Nothing concerning your rental property can be claimed until the date that property is "available for rent". That date is commonly the date you put the "FOR RENT" sign in the front year. It's also the date that a renter "could" have moved in. So if a renter could "not" have moved in on or before Dec 31, 2020, the property was flat out not "available for rent".
Additionally, since you had no rental income in 2020, there's nothing to deduct any 2020 rental expenses from. So any expenses incurred (and I seriously doubt you had any "allowed" expenses in 2020) wouldn't make one penny of difference to your 2020 tax liability.
The only "allowed" costs I 'know" you would have from 2020, would be any property improvements that you paid for *in* 2020. But you don't even report those costs until the tax year the property is actually "in service". Understand that property improvements and repairs are two physically different things. The below definitions should explain this. (Education is key here, so I'm trying to help you learn at least "the basics")
Based on the information you've provided thus far, I see no benefit (not a single penny) to putting anything concerning this on your 2020 tax return.
RENTAL PROPERTY ASSETS, MAINTENANCE/CLEANING/REPAIRS DEFINED
Property Improvement.
Property improvements are expenses you incur that “better” the property. Basically, they retain or add value to the property. Expenses for this are entered in the Assets/Depreciation section and depreciated over time. Property improvements can be done at any time after your initial purchase of the property. It does not matter if it was your residence or a rental at the time of the improvement. It still adds value to the property.
To be classified as a property improvement, two criteria must be met:
1) The improvement must become "a material part of" the property. For example, remodeling the bathroom, new cabinets or appliances in the kitchen. New carpet. Replacing that old Central Air unit.
2) The improvement must retain or add "real" value to the property. In other words, when the property is appraised by a qualified, certified, licensed property appraiser, he will appraise it at a higher value, than he would have without the improvements.
There are rules that allow you to just flat-out expense and deduct some property improvements, if the total cost of the improvement was less than $2,500. It’s referred to as “safe harbor di-minimis” But depending on the specific situation, this may or may not be beneficial. Just be aware that not every property improvement that cost less than $2,500 qualifies for this. If this interest you, the rules can get complex. So a good place to start reading is on the IRS website at https://www.irs.gov/businesses/small-businesses-self-employed/tangible-property-final-regulations. The stuff on di-minimis starts about one page down.
Cleaning & Maintenance
Those expenses incurred to maintain the rental property and it's assets in the useable condition the property and/or asset was designed and intended for. Routine cleaning and maintenance expenses are only deductible if they are incurred while the property is classified as a rental. Cleaning and maintenance expenses incurred in the process of preparing the property for rent are not deductible.
Repair
Those expenses incurred to return the property or it's assets to the same useable condition they were in, prior to the event that caused the property or asset to be unusable. Repair expenses incurred are only deductible if incurred while the property is classified as a rental. Repair costs incurred in the process of preparing the property for rent are not deductible.
Additional clarifications: Painting a room does not qualify as a property improvement. While the paint does become “a material part of” the property, from the perspective of a property appraiser, it doesn’t add “real value” to the property.
However, when you do something like convert the garage into a 3rd bedroom for example, making a 2 bedroom house into a 3 bedroom house adds “real value”. Of course, when you convert the garage to a bedroom, you’re going to paint it. But you will include the cost of painting as a part of the property improvement – not an expense separate from it.
