401k contribution

I am retired on pension and SS, 63 years old and have a 401k with my employer prior to my retirement. My wife has maxed contributions on her 401k for 2020. Turbo Tax shows that if I contribute $7000 to my 401k it could save me potentially $1500. Currently I have not made a contribution nor did so in 2020. My employer plan now that I am retired will not allow me to contribute further. My question is based on the above detail, can I open a "new" 401k account before April 15, contribute the $7000 and claim on my taxes for 2020 to receive this credit as noted in Turbo Tax program?